1.在BaseNavigationController中 ：

 UIImage *backButtonImage = [[UIImage imageNamed:@"返回"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
 self.navigationBar.backIndicatorImage = backButtonImage;
 self.navigationBar.backIndicatorTransitionMaskImage = backButtonImage;

2.在BaseViewController中：

 self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"" style:(UIBarButtonItemStylePlain) target:self action:nil];