diff --git a/HealthMonitor.Service/Cache/DeviceCacheManager.cs b/HealthMonitor.Service/Cache/DeviceCacheManager.cs
index 623d6b0..f9c7b2e 100644
--- a/HealthMonitor.Service/Cache/DeviceCacheManager.cs
+++ b/HealthMonitor.Service/Cache/DeviceCacheManager.cs
@@ -18,6 +18,7 @@ namespace HealthMonitor.Service.Cache
         private const string CACHE_KEY_GPSDEVICE_WATCH_CONFIG = "#GPSDEVICE_WATCH_CONFIG_HASH";
         private const string CACHE_KEY_FETALMOVEMENT = "FM_";
         private const string CACHE_KEY_FETALHEARTRATE = "FHR_";
+        private const string CACHE_KEY_PHRFREQSTATUS = "PhrFreqStatus_";
 
         private readonly GpsLocationHistoryAccessorClient<HisGpsFetalMovement> _hisFetalMovementApiClient;
         private readonly GpsLocationHistoryAccessorClient<HisGpsFetalHeartRate> _hisFetalHeartRateApiClient;
@@ -86,7 +87,7 @@ namespace HealthMonitor.Service.Cache
         {
             try
             {
-                var key = $"PhrFreqStatus_{sn}";
+                var key = CACHE_KEY_PHRFREQSTATUS+sn;
                 var status = await RedisHelper.GetAsync(key).ConfigureAwait(false);
                 if (string.IsNullOrEmpty(status)) return null;
                 return JsonConvert.DeserializeObject<PregnancyHeartRateModel>(status);
@@ -102,7 +103,7 @@ namespace HealthMonitor.Service.Cache
         {
             try
             {
-                var key = $"PhrFreqStatus_{sn}";
+                var key = CACHE_KEY_PHRFREQSTATUS + sn;
                 var data=JsonConvert.SerializeObject(status);
                 await RedisHelper.SetAsync(key, data, expire).ConfigureAwait(false);
             }
@@ -116,7 +117,7 @@ namespace HealthMonitor.Service.Cache
         {
             try
             {
-                var key = $"PregnancyHeartRateFreqStatus_{sn}";
+                var key = CACHE_KEY_PHRFREQSTATUS + sn;
                 await RedisHelper.DelAsync(key).ConfigureAwait(false);
             }
             catch (Exception ex)