We put forward a model of calculating the threshold gain of random laser, a method for obtaining the distribution function and the relevant random variables according to the mean free path and scattering intensity distribution. The distribution functions are derived for isotropic scattering of light, and then we get the random variables according with the distribution at every scattering. Simulating the random walk of photons in random media by using Monte Carlo method, the threshold of random laser is got. The relationship of the threshold gain to the photon mean free path, the radius of gain zone, the depth of gain zone and light absorption coefficient of non-gain zone is discussed. We make some suggestions to lower the threshold. The theoretical results agree with the experimental data.