Abstract: Avalanche photodiode (APD) have been widely applied in laser ranging systems for high sensitivity and broad bandwidth. Based on the avalanche effect of an APD, adding a local oscillator signal to the APD bias voltage, optoelectronic heterodyne mixing is to make down converter of signal into realization. In order to analyze the effect of APD optoelectronic heterodyne mixing on signal-to-noise ratio (SNR) of a system, the mathematical model of an APD was built. The relationship between direct-current component of the bias voltage and SNR and the effect of APD breakdown voltage change on SNR were analyzed by simulation. The results show that SNR of the APD output signal is reduced by optoelectronic heterodyne mixing, however, the bandwidth of subsequent circuit is reduced too, which can improve SNR of the whole system. When the input signal of an APD is weak, optoelectronic heterodyne mixing of the APD can improve the SNR of the system.