APD arrays provide an efficient method for photon detection probability improvement. However, the noise detection probability increases as well as the echo detection probability. Properly choosing the unit number of APD arrays means a lot for signal-to-noise ratio (SNR) improvement. In this article, according to the photon detection probability in the Geiger-mode, the SNR model with the unit number N was established based on the distribution of echoes and noise within the range gate. Effects of number of echoes, noise rate, location of echoes and fill factor were discussed. Analytical results show that larger number of echoes, higher fill factor and more precise orbit prediction help increase the SNR with APD arrays. 4-unit APD arrays are enough for laser ranging with echo number smaller than 0.1 and noise number within the range gate smaller than 1, while when there are large number of echoes and noise, 25-unit APD arrays will achieve a better SNR. The established SNR model for APD arrays will help for proper unit number selection to achieve the best SNR.