The limb infrared sounding is an effective means of detecting the space target with weak radiation. The infrared radiation characteristics of the limb background have an important influence on the target detection and tracking. The radiation statistical characteristics of the limb background were modeled by the measuring images: the background radiance had different distribution patterns in the vertical and horizontal directions of the horizon projection in the image. The vertical distribution was related to the tangent height. The horizontal direction could be approximated as a normal distribution, and the coefficient of variation ranged from 0.01 to 0.05. Based on the modeling results, a fast simulation method of limb background infrared image was proposed. The experimental results show that the simulation image and the measuring image have high consistency, which verifies the effectiveness of the imaging model and simulation method. The time of the fast simulation method is 1/200 of the conventional pixel-by-pixel simulation method, which can meet the requirements of real-time simulation. The research results can be applied to the fields of infrared camera design and target detection algorithm testing.