A kind of scattering is discussed and the corresponding distribution function is deduced. The geometry of the gain area is analyzed, the spatial distribution of output laser is obtained by simulating the movement of stochastic photon in random medium through the Monte Carlo method. The results show that the laser intensity is the strongest in the normal direction. It weakens gradually with the angle increasing. The light intensity is symmetrical on the normal. The spatial distribution of output laser is connected with the mean free path of photons in random medium. According to these, the mean free path of the photons can be obtained by measuring the spatial distribution of output laser. The majority of output laser photons (generally above 95%) scatter out within the distance of three mean free path from the interface.