diff --git a/SensorsAnalyticsSDK/SensorsAnalyticsSDK.podspec b/SensorsAnalyticsSDK/SensorsAnalyticsSDK.podspec
index cc26834f..3b9f12d8 100644
--- a/SensorsAnalyticsSDK/SensorsAnalyticsSDK.podspec
+++ b/SensorsAnalyticsSDK/SensorsAnalyticsSDK.podspec
@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "SensorsAnalyticsSDK"
-  s.version      = "1.5.4"
+  s.version      = "1.5.5"
   s.summary      = "The offical iOS SDK of Sensors Analytics."
   s.homepage     = "http://www.sensorsdata.cn"
   s.source       = { :git => 'https://github.com/sensorsdata/sa-sdk-ios.git', :tag => "v#{s.version}" } 
diff --git a/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.h b/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.h
index 777cdefb..484a3357 100755
--- a/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.h
+++ b/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.h
@@ -96,7 +96,7 @@ typedef NS_ENUM(NSInteger, SensorsAnalyticsDebugMode) {
  * 两次数据发送的最小时间间隔，单位毫秒
  *
  * @discussion
- * 默认值为 60 * 1000 毫秒， 在每次调用track、trackSignUp以及profileSet等接口的时候，
+ * 默认值为 15 * 1000 毫秒， 在每次调用track、trackSignUp以及profileSet等接口的时候，
  * 都会检查如下条件，以判断是否向服务器上传数据:
  * 1. 是否WIFI/3G/4G网络
  * 2. 是否满足以下数据发送条件之一:
diff --git a/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.m b/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.m
index 48f65389..7b70d50c 100755
--- a/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.m
+++ b/SensorsAnalyticsSDK/SensorsAnalyticsSDK/SensorsAnalyticsSDK.m
@@ -26,7 +26,7 @@
 #import "SASwizzler.h"
 #import "SensorsAnalyticsSDK.h"
 
-#define VERSION @"1.5.4"
+#define VERSION @"1.5.5"
 
 #define PROPERTY_LENGTH_LIMITATION 8191
 
@@ -202,7 +202,7 @@ - (instancetype)initWithServerURL:(NSString *)serverURL
         self.vtrackServerURL = vtrackServerURL;
         _debugMode = debugMode;
         
-        _flushInterval = 60 * 1000;
+        _flushInterval = 15 * 1000;
         _flushBulkSize = 100;
         _vtrackWindow = nil;
         
@@ -300,11 +300,13 @@ - (void)_flush {
                 NSString *errMsg = [NSString stringWithFormat:@"%@ network failure: %@", self, error];
                 SAError(@"%@", errMsg);
                 flushSucc = NO;
+                dispatch_semaphore_signal(flushSem);
                 return;
             }
             
             if (![response isKindOfClass:[NSHTTPURLResponse class]]) {
                 flushSucc = NO;
+                dispatch_semaphore_signal(flushSem);
                 return;
             }
             
@@ -326,6 +328,7 @@ - (void)_flush {
                 } else {
                     SAError(@"%@", errMsg);
                     flushSucc = NO;
+                    dispatch_semaphore_signal(flushSem);
                     return;
                 }
             } else {
@@ -440,7 +443,7 @@ - (void)_flush {
 #endif
         return YES;
     };
-    
+
     [self flushByType:@"Post" withSize:(_debugMode == SensorsAnalyticsDebugOff ? 50 : 1) andFlushMethod:flushByPost];
     [self flushByType:@"SFSafariViewController" withSize:50 andFlushMethod:flushBySafariVC];
     
@@ -1079,7 +1082,7 @@ - (void)setFlushInterval:(UInt64)interval {
     @synchronized(self) {
         _flushInterval = interval;
     }
-    [self flush];
+    [self _flush];
     [self startFlushTimer];
 }
 
@@ -1090,7 +1093,7 @@ - (void)startFlushTimer {
             double interval = _flushInterval > 100 ? (double)_flushInterval / 1000.0 : 0.1f;
             self.timer = [NSTimer scheduledTimerWithTimeInterval:interval
                                                           target:self
-                                                        selector:@selector(flush)
+                                                        selector:@selector(_flush)
                                                         userInfo:nil
                                                          repeats:YES];
         }
@@ -1194,7 +1197,7 @@ - (void)applicationDidEnterBackground:(NSNotification *)notification {
     SADebug(@"%@ application did enter background", self);
     
     if (self.flushBeforeEnterBackground) {
-        [self flush];
+        [self _flush];
     }
     
     if ([self.abtestDesignerConnection isKindOfClass:[SADesignerConnection class]]