A heterodyne optical head of a phase-shift laser range finder has been developed to measure distances with high accuracy. To increase the signal-to-noise ratio of the photoelectric current, an avalanche photodiode (APD) working as an optoelectronic mixer was used. A new behavioral model is proposed to describe the optical head in order to evaluate its performance according to the background light intensity. When the APD is biased with a dc current source, it is then demonstrated that the signal-to-noise ratio of the signal at the intermediate frequency, as well as the conversion gain, is inversely proportional to the average primary photoelectric current, i.e., to the background light collected by the APD. Yet, the performance obtained can be improved by using an optical filter. Furthermore, the background light also introduces a phase-shift error, which is evaluated so as to estimate the distance error.