Aiming at the mutual influence of signal, noise and multiplication factors of the avalanche photodiode under constant false alarm, the effects of the avalanche photodiode′s own noise and background radiation on the detection performance and the principle of constant false alarm noise compensation were analyzed. The relationship model of background radiation with the constant false alarm multiplication factor were established. The model was verified by experiments, and the relationship between background radiation and multiplication factors under different false alarm probability, threshold current, and noise current was simulated and analyzed. It is found that the multiplication factor would decrease with the increase of background radiation, and the decreasing rate gradually slows down. When the background radiation is constant, the increase of the false alarm probability or the threshold current will increase the multiplication factor. The sum of effective value of the thermal noise and the amplifier noise has a greater influence on the multiplication factor, while the dark current has less influence on the multiplication factor.