For small imaging detectors, the effectiveness of the automatic dimming method directly affects the subsequent target detection and tracking effect. A new adaptive dynamic dimming method based on embedded FPGA was proposed in this paper, which combined brightness histogram adaptive dimming method with image entropy search method. Because of the parallel computing advantage of FPGA, each function module could be running at the same time, and it met the need of real time. The key technologies in timing control circuit and imaging experiments were given. The experimental results show that the automatic dimming method has good environmental adaptability under a variety of illumination conditions.