To create a new UIWindow over the main window

UIWindow *window1 = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 320, 320)];
window1.backgroundColor = [UIColor redColor];
window1.windowLevel = UIWindowLevelAlert;
[window1 makeKeyAndVisible];

Finally I know why it doesn’t work, because window1 is a method var, and it will lost after the method executed. So I declare a new @property for it, as

@property (strong, nonatomic) UIWindow *window2;

and
change the code like

UIWindow *window2 = [[UIWindow alloc] initWithFrame:CGRectMake(0, 80, 320, 320)];
window2.backgroundColor = [UIColor redColor];
window2.windowLevel = UIWindowLevelAlert;
self.window2 = window2;
[window2 makeKeyAndVisible];

it works!