CGRect frame = f;//f表示你的textField的frame

 frame.size.width = ;//设置左边距的大小

 UIView *leftview = [[UIView alloc] initWithFrame:frame];

 textField.leftViewMode = UITextFieldViewModeAlways;//设置左边距显示的时机，这个表示一直显示

 textField.leftView = leftview;