Time-domian broadening is one of the very important features of laser pulse propagating underwater, which affects the performances of blue-green laser submarine communication system and underwater laser imaging system directly. The relationship between characteristic parameter of depth of gating (DOG) profile and that of laser echo profile and intensified change coupled device (ICCD) gating profile is deduced, which is the theoretical foundation of the proposed method to measure the time-domian broadening of underwater laser pulse. With the help of the high RPF laser underwater imaging system constructed by our term, the laser pulse time-domian broadening in large-scale tank is experimentally measured. The results show that the method proposed works well and is suitable for the conditions at long range.