In order to estimate the detection range of underwater laser imaging system, according to the imaging mechanism, the analysis of target radiation is reported, along with the analysis of seawater attenuation and other factors, and then the Signal-to-noise Ratio(SNR) model of the underwater laser imaging system is established. According to thethreshold of SNR for recognizing the target, along with the performance of laser and other devices, formulation of detection range is obtained. Then computation and simulation of detection range is implemented. By adopting a 532nm Nd:YAG laser, self-made ICCD camera and a range-gated sync control board based on FPGA technology, the underwater laser imaging experiment is done. The experimental results, which indicate that the average error of SNR between theoretical model and real image is 1.37dB, show the rationality of the SNR model.