The theoretical formulas of Gaussian beam describe its propagation characteristics on the whole, but they don not give any description of the propagation of photons within the beam. Based on Gaussian beam's characteristic of hyperboloid with one sheet, a new photon propagation model for Gaussian beam was built. The model takes the Gaussian beam as a hyperboloid consisting of myriads of co-axis hyperboloids of one sheet, and the propagation direction of a photon is chosen equiprobably between the two generatrices of the hyperboloid that goes through the photon's initial emission point. With this model and by use of tracing the propagations of lots of photons through an optical system, the parameters of Gaussian beam were obtained, i.e. geometric configuration, light intensity distribution, photons' optical path length distribution and their propagation directions. The computation result for a practical aspheric optical system has fully proved that the method can be used to simulate the propagation characteristics of Gaussian beam allsidedly and accurately with high numerical computation efficiency.