A model of solar irradiation on sensors onboard the FY-4 geostationary satellite in the case of solar intrusion was proposed. Solar radiation and the angle of incidence of the sun light on arbitrary surface of satellite can be calculated at any given time. Considering structure of baffle, the solar radiation, angle of incidence and the solar intrusion duration on scanning mirror of atmosphere vertical sounder on any day of the year can be given. The changes in one year are also given. The simulation results are similar with the measured data of GOES-8 (Geostationary Operational Environmental Satellites), and proved the accury of the simulation data.