diff --git a/slime/utils/train_infer_is.py b/slime/utils/train_infer_is.py
index c0379257a8..bceeb39a87 100644
--- a/slime/utils/train_infer_is.py
+++ b/slime/utils/train_infer_is.py
@@ -5,6 +5,16 @@
 from slime.backends.megatron_utils.cp_utils import all_gather_with_cp, slice_log_prob_with_cp
 
 
+def masked_sum(x: torch.Tensor, loss_mask: torch.Tensor, expand: bool = False) -> torch.Tensor:
+    result = (x * loss_mask).sum()
+    return result.expand_as(x) if expand else result
+
+
+def masked_mean(x: torch.Tensor, loss_mask: torch.Tensor, expand: bool = False) -> torch.Tensor:
+    result = masked_sum(x, loss_mask) / torch.clamp_min(loss_mask.sum(), 1)
+    return result.expand_as(x) if expand else result
+
+
 def metrics_append(metrics: Dict[str, list[torch.Tensor]], key: str, value: torch.Tensor) -> None:
     """
 
@@ -138,8 +148,9 @@ def compute_train_infer_is_weights(
 
     # handle each sequence independently
     for train_log_prob, rollout_log_prob, loss_mask in zip(train_log_probs, rollout_log_probs, loss_masks):
-        raw_log_ratio = train_log_prob - rollout_log_prob
         loss_mask = loss_mask.float()
+        add_ppl_metrics(train_log_prob, rollout_log_prob, loss_mask, metrics)
+        raw_log_ratio = train_log_prob - rollout_log_prob
 
         # level: The aggregation level for the importance sampling weights.
         if level == "token":
@@ -147,24 +158,21 @@ def compute_train_infer_is_weights(
             log_ratio_for_metrics = raw_log_ratio
         elif level == "sequence":
             # Product of ratios (unbiased but high variance)
-            agg_log_ratio = (raw_log_ratio * loss_mask).sum()
-            log_ratio_for_metrics = agg_log_ratio.expand_as(raw_log_ratio)
+            log_ratio_for_metrics = masked_sum(raw_log_ratio, loss_mask, expand=True)
         elif level == "geometric":
             # Geometric mean of ratios (biased but low variance)
-            agg_log_ratio = (raw_log_ratio * loss_mask).sum() / torch.clamp_min(loss_mask.sum(), 1)
-            log_ratio_for_metrics = agg_log_ratio.expand_as(raw_log_ratio)
+            log_ratio_for_metrics = masked_mean(raw_log_ratio, loss_mask, expand=True)
         else:
             raise ValueError(f"Invalid importance sampling level: {level}")
 
         log_ratio_safe = torch.clamp(log_ratio_for_metrics, min=-SAFETY_BOUND, max=SAFETY_BOUND)
         weights = torch.exp(log_ratio_safe)
+        metrics_append(metrics, "ratio_mean_before_tis", weights)
 
         # mask out catastrophic tokens
         if args.train_infer_is_veto_threshold is not None:
             veto_mask = calculate_veto_mask(raw_log_ratio, loss_mask, args.train_infer_is_veto_threshold, metrics)
 
-        metrics_append(metrics, "raw_ratio_mean", weights)
-
         # mode: how to handle the importance sampling weights exceeding the thresholds.
         if args.train_infer_is_mode == "truncate":
             # Cap the importance sampling weights at the upper threshold
@@ -261,3 +269,56 @@ def slice_cp_and_concat(
         is_metrics[key] = values
 
     return is_weights, is_metrics
+
+
+def add_ppl_metrics(
+    train_log_prob: torch.Tensor,
+    rollout_log_prob: torch.Tensor,
+    loss_mask: torch.Tensor,
+    metrics: Dict[str, list[torch.Tensor]],
+):
+    loss_mask = loss_mask.float()
+
+    # 1. Training policy perplexity metrics
+    mean_log_prob_training = masked_mean(train_log_prob, loss_mask, expand=True)
+    training_log_ppl = -mean_log_prob_training
+    training_ppl = torch.exp(training_log_ppl)
+    metrics_append(metrics, "training_log_ppl", training_log_ppl)
+    metrics_append(metrics, "training_ppl", training_ppl)
+
+    # 2. Rollout policy perplexity metrics
+    mean_log_prob_rollout = masked_mean(rollout_log_prob, loss_mask, expand=True)
+    rollout_log_ppl = -mean_log_prob_rollout
+    rollout_ppl = torch.exp(rollout_log_ppl)
+    metrics_append(metrics, "rollout_log_ppl", rollout_log_ppl)
+    metrics_append(metrics, "rollout_ppl", rollout_ppl)
+
+    # 3a. kl: Direct estimator for KL(π_rollout || π_training)
+    # This is the standard KL divergence: E[log(π_rollout) - log(π_training)]
+    # Positive value means rollout policy is more confident than training policy
+    kl_per_token = rollout_log_prob - train_log_prob
+    metrics_append(metrics, "kl", kl_per_token)
+
+    # 3b. K3 KL estimator for improved stability
+    # More stable for small KL values using: E[exp(log_ratio) - log_ratio - 1]
+    # Formula: KL ≈ E[r - log(r) - 1] where r = π_training/π_rollout
+    log_ratio = train_log_prob - rollout_log_prob
+    k3_kl_matrix = torch.exp(log_ratio) - log_ratio - 1
+    metrics_append(metrics, "k3_kl", k3_kl_matrix)
+
+    # 3c. Log PPL difference (sequence-level perplexity difference)
+    # log_ppl_diff = mean_log_prob_rollout - mean_log_prob_training
+    # Since ppl = exp(-log_prob), we have:
+    #   log(ppl_ratio) = log(training_ppl/rollout_ppl) = log_ppl_diff
+    # Positive value means training assigns lower probability (higher PPL) than rollout
+    log_ppl_diff = mean_log_prob_rollout - mean_log_prob_training
+    metrics_append(metrics, "log_ppl_diff", log_ppl_diff)
+    metrics_append(metrics, "log_ppl_abs_diff", log_ppl_diff.abs())
+    metrics_append(metrics, "log_ppl_diff_max", log_ppl_diff)
+    metrics_append(metrics, "log_ppl_diff_min", log_ppl_diff)
+
+    # 3d. PPL ratio (how much higher is training PPL vs rollout PPL)
+    # For numerical stability, compute in log space using log_ppl_diff
+    # Note: log_ppl_diff = log(ppl_ratio), so ppl_ratio = exp(log_ppl_diff)
+    ppl_ratio = torch.exp(log_ppl_diff)
+    metrics_append(metrics, "ppl_ratio", ppl_ratio)