To tackle the problems of low power efficiency and low contrast of liquid crystal display, a global dimming algorithm based on image classification is proposed. As brightness and contrast are two important display parameters, the paper takes use of these two parameters to classify the input images. Backlight dimming level is determined by image category, the average luminance and the difference between the average and maximum grayscales of all subpixels. The S-shaped curve is generated based on the characteristic value and the image category of input image to compensate liquid crystal pixels. For 60 images, the simulation results show that the average backlight power consumption is reduced by 19% and the static contrast ratio is improved by 143%. The prototype is developed based on the dimming algorithm. Compared with the LCD TV without applying the dimming algorithm, the practical results show that the average power consumption is reduced by 12.9% and the static contrast ratio is improved by 43.18%.