A heterodyne photo detection method is proposed in order to improve the signal to noise ratio of a frequency-modulated-continuous-wave (FMCW) laser range finder. For this purpose, the non-linearity of the current-voltage characteristic of an avalanche photodiode (APD) is used. The photoelectric current is mixed with a local oscillator (LO) signal inside the APD working as an optoelectronic mixer. The signal from the LO can be injected inside the APD by an electrical channel. Experimental results are in agreement with the theoretical analysis, and this method shows that it can greatly improve the measurement distance in phase-shift laser range finder with non-cooperative target.