When no monitor connect , the resolution can only be 1920 * 1080 in 30hz?

Are you connecting a monitor or a TV?

Below are some methods that could perhaps solve the problem, singly or several together :

Method 1 : Use a tool to force the resolution

Here are some command line tools that can change the screen resolution :
QRes, NirCmd, Display Changer.

Method 2 : Registry updates

Search the registry for DefaultSettings.XResolution, DefaultSettings.YResolution and BitsPerPixel and change to the desired resolution.

If necessary, add ColorDepth for RDP connection :

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services] "ColorDepth"=dword:00000003 in decimal.

More info can be found here.

Method 3 : Setting the video adapter to custom setting

If your VGA video driver has customizing software, you could do something similar to:

Method 4 : Use a fake video plug

If it becomes necessary to fool Windows into believing that a real monitor is attached, see this tutorial on how to create such a plug using a spare DVI-VGA adapter and three 68 ohm resistors: The 30 Second Dummy Plug.

Method 5 : Use PowerStrip (shareware, $29.95)

The PowerStrip utility allows you to gain complete control over the video card.
For more details see : Customize monitor resolution settings with PowerStrip.