Based on space applications, avalanche photodiode (APD) optimal gain control was investigated in terms of space borne laser range finder. By introducing current SNR(signal to noise ratio) model, the key factors were analyzed which mainly affects SNR of APD in space borne laser range finder. According to special application of space borne laser range finder, optimal gain control of APD was studied. Temperature feedback control circuit for APD optimal gain was designed, temperature digital feedback control algorithm for APD optimal gain was deduced, and the validity and temperature adaptability of the control circuit and the algorithm were demonstrated. It is shown from the experiment that, because of the designed control circuit and algorithm, the gain factor of APD maintains is constant in the temperature range from -25 ℃ to 60 ℃, which meets the requirement for space borne applications.