Abstract: Range-gated underwater laser imaging system is a kind of underwater photoelectric imaging system which is based on the principle of time label. It can effectively restrain the backscatter of water and improve the range of target detection, and has superior value for military and civil use. In the range-gated underwater laser imaging system, the lights received by ICCD include the backscatter of water, the direct component and forward scatter of reflected light of the target. Computing model of light energy received by ICCD was built. Giving the moment and the width of ICCD opening gate, the direct component and forward scatter of reflected light were analyzed and the calculation methods based on PSF theory were studied. The backscatter of water was calculated through water layering. According to those studies, the signal to noise ratio of range-gated underwater laser imaging system was given. Finally, the impact of target range on the SNR was analyzed and simulation result was given.