The constant false alarm rate (CFAR) controlling technique of avalanche photo diode (APD) applied in Lidar was studied, and a CFAR controlling circuit based on FPGA was described. Our results show that it is capable of self-adjusting according to the background radiation by means of noise detection, and it can also maintain constant FAR and make the APD in the optimum state of bias voltage. Experimental results indicate that this circuit can well realize CFAR controlling.