Signal to noise ratio(SNR)is an important characteristic of active laser detection system.The signal-noise ratio model of APD is established by analyzing the noise sources of APD and the reflected power of “cat eye” target with focal shift.The influence factors such as focal shift,detection distance and laser divergence angle to the SNR are analyzed.Then the effects are simulated through numerical computation.The results demonstrate that the reflected power of a “cat eye” target will attenuate as the focal shift increases which consequently result in the decrease of SNR.The SNR of APD will also decrease with the increasing of the distance of “cat eye” target and laser divergent angle.The study will lay the foundation for the design and performance evaluation of active laser detection systems based on APD.