In order to meet the demand for accurate and rapid multiple scattering simulation of fog, when the short-range radar distance was close, a fast and efficient simulation method of multi-scattering was proposed based on the combination of Mie theory and radar equation. First, the fog model was built according to the fog particle size distribution. Then, scattering functions about single particle were given based on Mie theory, and the first order scattering and the second order scattering power densities of single particle in fog model were calculated. The expressions of multiple scattering cross section of single particle and scattering coefficients of fog were deduced based on radar equation. Simulation results show that the second order scattering cross section is approximately proportional to 2/3 power density of fog, and the second order scattering coefficient with increasing visibility decreases more quickly than the first order scattering.