1. A method to automatically regulate brightness of liquid crystal displays, said method comprising the steps of: enabling a gray level of an output image to be regulated by an image brightness regulating unit, said image brightness regulating unit being comprised of a memory unit and a multiplier, based on ambient light and images, said gray level of output image being regulated in such a manner to store images in said memory unit, after completion of statistics of image brightness ratio and identification of brightness gains, said images in said memory unit being output with red, green and blue gray levels of the images multiplied by corresponding brightness gains, thereby regulating the image brightness by changing image gray level; and applying an image brightness ratio computing and output controlling unit for statistical analysis of image brightness, said image brightness ratio computing and output controlling unit comprising: a gray level/brightness converting unit, enabling conversion between gray level and corresponding brightness of image, such that a linearity between gray level and brightness accurately calculates brightness of images; a pixel brightness calculating unit converting red, green and blue gray levels of pixel into brightness; a display brightness statistical unit used to calculate total brightness Y image and brightness ratio BR of every display, with addition of the brightness of every point in the display so as to obtain total brightness of the display, the total brightness of the display being divided by maximum total brightness Y total of the image, obtaining a brightness ratio of an image; a brightness gains unit being comprised of a brightness gains table, corresponding brightness gains being checked using a brightness ratio, said brightness gains table being a relational table of brightness gains and a brightness ratio established based on a relationship between expected brightness and brightness ratio; a gains regulation determining unit used to calculate and regulate change rate of a brightness gain, the brightness gain output by the brightness gains unit varying with a sharp change of brightness ratio, the brightness gains being output, the brightness gain varying little with slow or minor change of a brightness ratio; an ambient light detection and brightness adaptation regulating unit, detecting the change of ambient light using an optical sensor; with a reference to LUT after low pass filtering of the change of ambient light, and determining the brightness gain of ambient light, the sensor of ambient light being available in monocolor or multicolor type, for a multicolor light sensor, the gains being properly regulated with color change of ambient light; and a gray level expanding unit aimed to avoid false contour of images arising from lower gray level after brightness gain operation, applying spacial error diffusion or time dithering technology to improve the hierarchical images of the lower gray level.