diff --git a/xllm/core/framework/block/block_manager_impl.cpp b/xllm/core/framework/block/block_manager_impl.cpp
index 17efb6e16..f006fbb23 100644
--- a/xllm/core/framework/block/block_manager_impl.cpp
+++ b/xllm/core/framework/block/block_manager_impl.cpp
@@ -20,6 +20,31 @@ limitations under the License.
 
 #include "framework/prefix_cache/prefix_cache_factory.h"
 namespace xllm {
+namespace {
+
+bool mark_used(std::vector<uint8_t>* usage_ids, int32_t block_id) {
+  CHECK(usage_ids != nullptr);
+  CHECK_GE(block_id, 0);
+  CHECK_LT(static_cast<size_t>(block_id), usage_ids->size());
+  if ((*usage_ids)[block_id] != 0) {
+    return false;
+  }
+  (*usage_ids)[block_id] = 1;
+  return true;
+}
+
+bool clear_used(std::vector<uint8_t>* usage_ids, int32_t block_id) {
+  CHECK(usage_ids != nullptr);
+  CHECK_GE(block_id, 0);
+  CHECK_LT(static_cast<size_t>(block_id), usage_ids->size());
+  if ((*usage_ids)[block_id] == 0) {
+    return false;
+  }
+  (*usage_ids)[block_id] = 0;
+  return true;
+}
+
+}  // namespace
 
 BlockManagerImpl::BlockManagerImpl(const Options& options)
     : BlockManager(options) {
@@ -34,6 +59,9 @@ BlockManagerImpl::BlockManagerImpl(const Options& options)
   size_t total_blocks = options_.num_blocks();
   block_size_ = options_.block_size();
   num_free_blocks_.store(total_blocks, std::memory_order_relaxed);
+  if (options_.enable_prefix_cache()) {
+    usage_accounted_ids_.assign(total_blocks, 0);
+  }
   free_blocks_.reserve(total_blocks);
   for (int32_t i = 0; i < total_blocks; ++i) {
     // push smaller block ids to the back of the vector
@@ -57,6 +85,10 @@ std::vector<Block> BlockManagerImpl::allocate(size_t num_blocks) {
     size_t prev_count =
         num_free_blocks_.fetch_sub(1, std::memory_order_relaxed);
     const int32_t block_id = free_blocks_[prev_count - 1];
+    if (options_.enable_prefix_cache()) {
+      CHECK(mark_used(&usage_accounted_ids_, block_id))
+          << "block " << block_id << " usage accounted repeatedly";
+    }
     blocks.emplace_back(block_id, this);
   }
 
@@ -69,7 +101,8 @@ void BlockManagerImpl::deallocate(const Slice<Block>& blocks) {
   if (options_.enable_prefix_cache()) {
     for (const auto& block : blocks) {
       // the block is not shared by other sequence
-      if (block.is_valid() && block.ref_count() <= 2) {
+      if (block.is_valid() && block.ref_count() <= 2 &&
+          clear_used(&usage_accounted_ids_, block.id())) {
         if (num_used_blocks_ == 0) {
           LOG(ERROR) << "num_used_blocks_==0 cannot fetch_sub for id:"
                      << block.id()
@@ -138,8 +171,7 @@ std::vector<Block> BlockManagerImpl::allocate_shared(
 
     // update effective block usage
     for (const auto& block : shared_blocks) {
-      // the block is not shared by any sequence
-      if (block.ref_count() <= 2) {
+      if (mark_used(&usage_accounted_ids_, block.id())) {
         num_used_blocks_.fetch_add(1, std::memory_order_relaxed);
       }
     }
@@ -187,6 +219,11 @@ Block BlockManagerImpl::allocate() {
 void BlockManagerImpl::free(int32_t block_id) {
   // do nothing for reserved block 0
   if (block_id != 0) {
+    if (options_.enable_prefix_cache() &&
+        clear_used(&usage_accounted_ids_, block_id)) {
+      CHECK_GT(num_used_blocks_.load(std::memory_order_relaxed), 0u);
+      num_used_blocks_.fetch_sub(1, std::memory_order_relaxed);
+    }
     size_t prev_count =
         num_free_blocks_.fetch_add(1, std::memory_order_relaxed);
     CHECK(prev_count < free_blocks_.size());
diff --git a/xllm/core/framework/block/block_manager_impl.h b/xllm/core/framework/block/block_manager_impl.h
index 8dc5540eb..13454e851 100644
--- a/xllm/core/framework/block/block_manager_impl.h
+++ b/xllm/core/framework/block/block_manager_impl.h
@@ -112,6 +112,9 @@ class BlockManagerImpl : public BlockManager {
 
   // free block list
   std::vector<int32_t> free_blocks_;
+
+  // Whether a block is already counted in num_used_blocks_.
+  std::vector<uint8_t> usage_accounted_ids_;
 };
 
 }  // namespace xllm
diff --git a/xllm/core/scheduler/prefill_only_scheduler.cpp b/xllm/core/scheduler/prefill_only_scheduler.cpp
index 0821edc65..92082b6fe 100644
--- a/xllm/core/scheduler/prefill_only_scheduler.cpp
+++ b/xllm/core/scheduler/prefill_only_scheduler.cpp
@@ -673,4 +673,4 @@ std::vector<Batch> PrefillOnlyScheduler::prepare_batch() {
   return batches;
 }
 
-}  // namespace xllm
\ No newline at end of file
+}  // namespace xllm