diff --git a/llmage/accounting.py b/llmage/accounting.py
index e076826..ef9087a 100644
--- a/llmage/accounting.py
+++ b/llmage/accounting.py
@@ -1,7 +1,7 @@
 import asyncio
 import json
 import time
-from datetime import datetime
+from datetime import datetime, timedelta
 from appPublic.log import exception, debug
 from appPublic.uniqueID import getID
 from appPublic.dictObject import DictObject
@@ -239,20 +239,18 @@ async def llm_accoung_failed(luid, reason=None):
 			await sor.C('llmusage_accounting_failed', failed_rec)
 
 
-async def backup_accounted_llmusage():
-	"""Backup accounted records with use_date before today to history table."""
+async def backup_accounted_llmusage(cutoff_date):
+	"""Backup accounted records with use_date < cutoff_date to history table."""
 	env = ServerEnv()
-	today = datetime.now().strftime('%Y-%m-%d')
 	ts = env.timestampstr()
 	batched = 0
 	async with get_sor_context(env, 'llmage') as sor:
-		# Select records with use_date < today (i.e. yesterday and earlier)
 		sql = """select * from llmusage 
 where accounting_status='accounted' 
-	and use_date < ${today}$"""
-		recs = await sor.sqlExe(sql, {'today': today})
+	and use_date < ${cutoff_date}$"""
+		recs = await sor.sqlExe(sql, {'cutoff_date': cutoff_date})
 		if not recs:
-			debug(f'backup_accounted_llmusage: no records to backup for use_date < {today}')
+			debug(f'backup_accounted_llmusage: no records to backup for use_date < {cutoff_date}')
 			return 0
 		debug(f'backup_accounted_llmusage: {len(recs)} records to backup')
 		for r in recs:
@@ -280,7 +278,7 @@ where accounting_status='accounted'
 			# Delete from main table
 			await sor.D('llmusage', {'id': r.id})
 			batched += 1
-	debug(f'backup_accounted_llmusage: backed up {batched} records')
+	debug(f'backup_accounted_llmusage: backed up {batched} records for use_date < {cutoff_date}')
 	return batched
 
 
@@ -335,14 +333,13 @@ order by failed_time desc limit {page_size} offset {offset}"""
 async def backend_accounting():
 	env = ServerEnv()
 	debug(f'backend accounting started ...')
-	backup_counter = 0
+	last_backup_date = None
 	while True:
 		try:
 			lus = await get_accounting_llmusages()
 		except Exception as e:
 			exception(f'{e}')
 			lus = []
-		# debug(f'{len(lus)=} need to accounting........')
 		for lu in lus:
 			try:
 				tpac = await get_user_tpac(lu.userid)
@@ -356,12 +353,14 @@ async def backend_accounting():
 				exception(f'{e}, {lu.id=}')
 				await llm_accoung_failed(lu.id, reason=str(e))
 
-		# Run backup every 30 iterations (~5 minutes)
-		backup_counter += 1
-		if backup_counter >= 30:
-			backup_counter = 0
+		# Check if date changed, trigger backup once per day
+		today = datetime.now().strftime('%Y-%m-%d')
+		if today != last_backup_date:
+			yesterday = (datetime.now() - timedelta(days=1)).strftime('%Y-%m-%d')
+			last_backup_date = today
 			try:
-				await backup_accounted_llmusage()
+				debug(f'date changed to {today}, triggering backup for use_date < {yesterday}')
+				await backup_accounted_llmusage(yesterday)
 			except Exception as e:
 				exception(f'backup_accounted_llmusage failed: {e}')
 
diff --git a/models/llmusage.json b/models/llmusage.json
index 7d0f6ca..3371734 100644
--- a/models/llmusage.json
+++ b/models/llmusage.json
@@ -123,6 +123,14 @@
             "idxfields": [
                 "userid"
             ]
+        },
+        {
+            "name": "idx_llmusage_accounting",
+            "idxtype": "index",
+            "idxfields": [
+                "accounting_status",
+                "use_date"
+            ]
         }
     ]
 }
\ No newline at end of file
diff --git a/scripts/migrate_llmusage_history.sql b/scripts/migrate_llmusage_history.sql
index 5c30e0c..9bebf84 100644
--- a/scripts/migrate_llmusage_history.sql
+++ b/scripts/migrate_llmusage_history.sql
@@ -69,6 +69,9 @@ CREATE INDEX idx_laf_llmid ON llmusage_accounting_failed(llmid);
 CREATE INDEX idx_laf_handled ON llmusage_accounting_failed(handled);
 CREATE INDEX idx_laf_failed_time ON llmusage_accounting_failed(failed_time);
 
+-- 3. 为 llmusage 表添加组合索引（优化备份查询: accounting_status + use_date）
+CREATE INDEX idx_llmusage_accounting ON llmusage(accounting_status, use_date);
+
 -- ============================================================
 -- 验证步骤（执行后运行）:
 -- 1. 确认表创建成功: