diff --git a/CustomKeyboard/View/KBFunctionBarView.m b/CustomKeyboard/View/KBFunctionBarView.m
index 27ba738..655c561 100644
--- a/CustomKeyboard/View/KBFunctionBarView.m
+++ b/CustomKeyboard/View/KBFunctionBarView.m
@@ -18,6 +18,7 @@
 @end
 
 @implementation KBFunctionBarView
+static const CGFloat kKBBackButtonWidth = 40;
 
 - (instancetype)initWithFrame:(CGRect)frame{
     if (self = [super initWithFrame:frame]) {
@@ -83,14 +84,14 @@
     UIButton *appButton = [UIButton buttonWithType:UIButtonTypeCustom];
     appButton.tag = 100; // 左侧 index = 0
     UIImage *appImage = [UIImage imageNamed:@"App_icon"];
-    [appButton setImage:appImage forState:UIControlStateNormal];
+    [appButton setBackgroundImage:appImage forState:UIControlStateNormal];
     appButton.imageView.contentMode = UIViewContentModeScaleAspectFit;
     appButton.adjustsImageWhenHighlighted = YES;
     [appButton addTarget:self action:@selector(onLeftTap:) forControlEvents:UIControlEventTouchUpInside];
     [self.leftContainer addSubview:appButton];
     [appButton mas_makeConstraints:^(MASConstraintMaker *make) {
         make.center.equalTo(self.leftContainer);
-        make.width.height.mas_equalTo(34); // 设计图尺寸
+        make.width.height.mas_equalTo(kKBBackButtonWidth); // 设计图尺寸
     }];
     self.leftButtonsInternal = @[appButton];