An advanced model of underwater laser range-gated imaging system is proposed in order to improve the shortcoming of present modeling methods in shoot-receive scheduling and instruments characteristic. One-dimensional (1D) rectangular signal is selected as the target, whose output signal to noise ratio (SNR) is used to evaluate the system performance. The time-domain broadening of pulse underwater and noise characteristic of intensified charge coupled device (ICCD) is integrated into the proposed model to improve the shortcoming of present models. The validity of the proposed model is validated through comparison of the simulation and the accquired images by the underwater laser range-gated imaging system. Finally the range-gated technology detects the target with middle and small size effectively with the system in different water qualities and maximum detecting distances, which shows that discovering, identifying and recognizing distances of the ranged-gated technology are at 9, 7.5 and 7 attenuation lengths respectively.