A grayscale histogram statistic method was proposed, a frame image histogram statistics was completed using FPGA and the next frame of image data conversion was made by the adaptive linear enhanced algorithm. According to the needs for real time processing, field programmable gate arrays (FPGA ) was applied to construct the highspeed image preprocessor and to finish the realtime calculation for infrared tracking image preprocessing. The simulation and experimental results demonstrate that this method is feasible, and it has the advantages such as: the simple code, less system resources occupied by FPGA, and the capability of realtime data conversion.