The working principle of Geiger-mode avalanche photodiode (GAPD) is analyzed, and then the activating model of GAPD via splitting the sample time into even time bin is established. Based on this result, the photon counting detection probability and noise model are enlisted, and according to the noise model the probability of false alarm (PFA) is calculated. The models of signal returned from diffuse target and glint target, are analyzed, finally the detection-probability model of photon counting combined with the noise and probabiliy of detection under the threshold are obtained.