The temporal spreading behavior of Gaussian laser pulses propagating through atmospheric turbulence is similar to the behavior of Gaussian pulses passing through Gaussian filter. Hence, a temporal spreading model, being a Gaussian filter approximated by cascading Butterworth filters in essence, is proposed for laser communication channel simulation. The 3 dB frequency point of the approximated Gaussian filter is closely associated with the physical parameters of atmospheric turbulence, including the structural constant of refractive index as well as the outer scale size of turbulence, and propagation path length. So the spreading of the Gaussian pulses is combined with the parameters of atmospheric turbulence, and an analytic expression for modeling the spreading is obtained. This model and the analytic expression represented by the model′s parameters are not only valid in weak fluctuation regime but also in weak-moderate-strong regime.