Based on the radiative transfer and Mie scattering theory, the computing method of analyzing the effects of forward scattering of fog on range-gated imaging system is given. Firstly, the empirical relation of water content and visibility for fog is used to estimate fog drop size distribution. Secondly, the Mie scattering phase function is presented. And then the apparent extinction coefficient can be calculated. Finally, the forward scattering, backscattering and signal irradiance on photo-cathode varying with time and the image contrast between object and background are obtained when the effects of forward scattering are considered. The results demonstrate that the irradiance on object, backscattering and signal irradiance on photo-cathode are increased and the image contrast between object and background is decreased. During the gated time, the irradiance of forward scattering on photo-cathode is much less than backward scattering.