To achieve auto-exposure in digital cameras, image brightness is widely used because of its direct relationship with exposure time. Although image entropy which represents information measure of image is widely used in various image processing applications, it has not been used on FPGA in AE system due to its complex calculation. Image with maximum entropy value contains more information. So this paper presents an algorithm based on image entropy. By searching maximum entropy value, the image can get an appreciate exposure. What’s more, by using formula manipulation of image entropy and piecewise linearization of the log function, the optimized algorithm grasps the overall change rule in stead of traditional calculation and has been realized on FPGA. The experiment results show that on basis of 10M work frequency of CMOS image sensor (electronic shutter, 1024 by 1024 pixels, on-chip 12-bit ADC) and 100M clock frequency of FPGA (ALTERA-EP2S60F48414N), this algorithm works well. And at the same time, the algorithm improves the amount of image information and increases the accuracy of auto exposure.