The Monte Carlo model of underwater photon propagation is developed to simulate the time-domain broadening characteristics of laser pulse propagating underwater.The influence of system parameters,such as propagation distance,receiving area,and water quality to the pulse time-domain broadening characteristics is analyzed.The simulated results show that with increasing of propagating distance,the laser pulse time-domain width increases.And the larger the receiving area of the detector,the more seriously the pulse time-domain broadens.Furthermore,the width of pulse time-domain is relatively narrow when the forward scattering characteristics of seawater is strong.And the pulse time-domain broadening is more clearly apparent with increasing attenuation coefficient.