The invention provides an automatic dimming method of a vehicle information system. The method includes the following steps: determining the maximum dimming parameter and the minimum dimming parameter of the system; setting dimming levels of a dimming interface; determining the maximum value and the minimum value of a first adjustable range of dimming parameters in a daytime mode; determining the maximum value and the minimum value of a second adjustable range of the dimming parameters in a night mode; determining current dimming modes, and calculating corresponding dimming parameters according to the corresponding dimming modes; and adjusting the brightness of a screen according to the dimming parameters. The dimming parameters of the vehicle information system are determined automatically, and the development efficiency is increased.