diff --git a/src/query/service/src/pipelines/builders/builder_merge_into.rs b/src/query/service/src/pipelines/builders/builder_merge_into.rs
index 030dfde08f687..57f055cc615dd 100644
--- a/src/query/service/src/pipelines/builders/builder_merge_into.rs
+++ b/src/query/service/src/pipelines/builders/builder_merge_into.rs
@@ -342,8 +342,11 @@ impl PipelineBuilder {
         } = merge_into_source;
 
         self.build_pipeline(input)?;
-        self.main_pipeline
-            .try_resize(self.ctx.get_settings().get_max_threads()? as usize)?;
+        self.main_pipeline.try_resize(
+            self.ctx
+                .get_settings()
+                .get_merge_into_resize_parallel_threads()? as usize,
+        )?;
         // 1. if matchedOnly, we will use inner join
         // 2. if insert Only, we will use right anti join
         // 3. other cases, we use right outer join
diff --git a/src/query/settings/src/settings_default.rs b/src/query/settings/src/settings_default.rs
index e2166e19584e1..e7a913ee3e066 100644
--- a/src/query/settings/src/settings_default.rs
+++ b/src/query/settings/src/settings_default.rs
@@ -498,6 +498,12 @@ impl DefaultSettings {
                     mode: SettingMode::Both,
                     range: Some(SettingRange::Numeric(0..=1)),
                 }),
+                ("merge_into_resize_parallel_threads", DefaultSettingValue {
+                    value: UserSettingValue::UInt64(0),
+                    desc: "tune the source parallel when too many small blocks",
+                    mode: SettingMode::Both,
+                    range: Some(SettingRange::Numeric(0..=u64::MAX)),
+                }),
                 ("enable_distributed_merge_into", DefaultSettingValue {
                     value: UserSettingValue::UInt64(0),
                     desc: "Enables distributed execution for 'MERGE INTO'.",
diff --git a/src/query/settings/src/settings_getter_setter.rs b/src/query/settings/src/settings_getter_setter.rs
index 9a1937342c5f8..6dba871767aa8 100644
--- a/src/query/settings/src/settings_getter_setter.rs
+++ b/src/query/settings/src/settings_getter_setter.rs
@@ -443,6 +443,13 @@ impl Settings {
         Ok(self.try_get_u64("enable_distributed_copy_into")? != 0)
     }
 
+    pub fn get_merge_into_resize_parallel_threads(&self) -> Result<u64> {
+        match self.try_get_u64("merge_into_resize_parallel_threads")? {
+            0 => Ok(self.get_max_threads()?),
+            value => Ok(std::cmp::min(self.get_max_threads()?, value)),
+        }
+    }
+
     pub fn get_enable_experimental_merge_into(&self) -> Result<bool> {
         Ok(self.try_get_u64("enable_experimental_merge_into")? != 0)
     }