Aiming at the interaction between noise, signal and multiplication factor under the working mode of constant false alarm, the effects of multiplication factor and background radiation on the performance of APD detection system and the working principle of constant false alarm compensation mode were analyzed. And the relationship model between background radiation and multiplication factor under constant false alarm is established. By evaluating the minimum signal power that can be detected with or without background light under certain conditions, an evaluation model of APD performance was established, and the model was verified by experiments. The influence of different noise on APD detection performance was analyzed. The results show that as the background light increases, the influence of background light on APD performance increases gradually, and the growth rate gradually decreases. When the dark current noise is small, the background light has a more obvious effect on APD. With the increase of dark current noise, the influence of background light decreases rapidly and eventually stabilizes. The effect of light on APD increases uniformly with the increase of thermal noise and noise of the amplifier circuit.