Abstract: In the application of using lidar to detect undersea targets,a kind of signal processing technology based on frequency filtering is proved to be able to suppress the seawater backscattering effectively.The bandwidth of the filter in the receiving system doesn't only suppress the backscattering,but also supresses the target signal.In order to estimate the range of the bandwidth,the seawater transport model of the modulated-pulse lidar signal,and the frequency characteristic of the seawater backscattering signal and target reflecting signal were studied.After simulating the received signal on frequency domain,the optimum design method on the bandwidth of the filter in the receiving system was found and the optimum signal-to-noise ratio of the system could be established by this result.