UINavigationBar *navigationBar = self.navigationController.navigationBar;

    // white.png图片自己下载个纯白色的色块，或者自己ps做一个

    [navigationBar setBackgroundImage:[UIImage imageNamed:@"nav2"]

                       forBarPosition:UIBarPositionAny

                           barMetrics:UIBarMetricsDefault];

    [navigationBar setShadowImage:[UIImage new]];