Due to the uneven distribution of the underwater laser transmission channels, the transmission beam will be affected by water absorption and scattering effects, which induce inter-symbol interference and the degradation of link performance. Based on the binomial Henyey–Greenstein scattering phase function and considering the forward and backward effects, the pulse response expression of the non-line-of-sight underwater laser channel is derived. The effectiveness of the underwater photon transmission channel model established is simulated and verified by the Monte Carlo method. The fitting effects in the coastal waters between the theoretical calculation value of channel impulse response and simulation are better. Under the same simulation parameters, the pulse-broadening factor in coastal waters is higher than that in harbor waters, and the time-domain broadening effect of laser pulse transmission in harbor waters is large. When the asymmetry factor is large, the influence of front scattering on pulse broadening is greater.