Laser pulse is the main signal of submarine optical wireless communication at present. To simulate the time-domain broadening of laser pulse propagating underwater, the small-angle approximation method, as a simple and inexpensive one, is presented in this paper. The classical electron scattering theory is applied to the photo scattering effects. Based on L. B. Stotts model and the appropriate water volume scattering function, an expression of laser pulse waveform is derived, and the widths of pulse propagating different distances are simulated under certain water quality conditions. The simulated results are in agreement with experimental measurement data and their precision, for longer-distance propagation, is higher than that of traditional methods, which can demonstrate the validity of this method, and provide an important basis for dealing with the problem of inter-symbol interference of longer-distance laser communication underwater.