Underwater range-gated imaging radar can not only eliminate back-scattering noise effectively, but also increase the underwater imaging operation distance. Imaging blind area limit is one of the main defects of the method for inherent delay of system signal processing. An asynchronous control method based on laser pulse is proposed to perform comparing experiments on high-repetition-rate and low-repetition-rate systems. Experimental results show that imaging blind area limit can be removed successfully from underwater high-repetition-rate range-gated imaging system by asynchronous control method, but this method would bring gray level jitter to low-repetition-rate system. It is one of the reason that the characteristics of underwater high-repetition-rate range-gated imaging system are better than that of low-repetition-rate system.