The transmission character of forward-scattering, backscattering and signal irradiance is studied. An underwater range-gated imaging system is built up with pulse-laser and ICCD. A mathematic method of underwater laser range-gated imaging system is given. The relation between imaging contrast and gate-controlled signal is analyzed. The theoretical calculations are programmed, and figures of image contrast and gate-controlled signal are plotted based on typical system programmed. A best strategy of gate-controlling is raised. Relatively, the experiment has been done. The results show that accurate open-time can acquire the best images, and the image is better when open the gate little-later than ahead with the ICCD not saturated. The duration of gating should one to there times of the pulse width.