Abstract: It mainly studies the influence of the flow field of the warhead on the irradiance of its infrared seeker during the hypersonic flight. A missile warhead and aerodynamic flow field model are established, and the flow field grid, the infrared seeker mirror grid, and the radiation field calculation grid are divided. Starting from the theoretical formulas of radiance and luminance, the integral formula of gas radiation suitable for radiant grid is deduced, and the flow field grid is interpolated into radiant grid. The gas radiation of the missile head flow field determined by the missile flying state is obtained by calculating the radiation of each node, and the radiation illuminance of the optical system mirror of the infrared seeker is obtained by integrating the radiation into the optical system mirror of the infrared seeker. The distribution law of the radiant illuminance of the main mirror of the seeker caused by the gas in the flow field is obtained:the center of the main mirror is the largest, and it is circular around the center of the main mirror. It decreases gradually along the radius direction, and the influence increases with the increase of Mach number.