从此回答

[self.navigationController.navigationBar setBackgroundImage:[UIImage new]
                     forBarMetrics:UIBarMetricsDefault];
self.navigationController.navigationBar.shadowImage = [UIImage new];
self.navigationController.navigationBar.translucent = YES;
self.navigationController.view.backgroundColor = [UIColor clearColor];
self.navigationController.navigationBar.backgroundColor = [UIColor clearColor];

此外，如Josh在注释中建议的，将bar设置为默认值：

Also, as suggested by Josh in the comments, to put the bar back to default:

[self.navigationController.navigationBar setBackgroundImage:nil
                     forBarMetrics:UIBarMetricsDefault];