Avalanche photodiode (APD) is the photoelectric detector of frequency modulated continuous wave (FMCW) radar and can realize mixing functions, its self-mixing technology involved in signal receiving and processing of FMCW radar is one of the core problem restricting its performance. In the process of achieving photoelectric mixing, the space charge effect have an impact on APD gain. On the basis of APD empirical gain model and theoretical analysis of space charge effect, an improvement is implemented to the original model. Experimental studies have demonstrated that the improved APD gain model is correct and the APD gain can be affected by space charge resistance, series load resistance and total current in the situation of strong light. Thereby the influencing factors of APD mixing performance as the photoelectric detector in FMCW laser radar are confirmed.