IR scene simulation is an important way for designing and assessing the IR guided missile，and the key of simulation is the generation of IR scene image.A method of generating sky background IR images based on fractal technology is proposed.The infrared models of sky and clouds are constructed based on physics analysis.The irradiance of sky background can be computed according to the viewpoint.To simulate infrared image of sky and clouds,irradiances of sky and clouds are mapped to gray levels,and clouds are generated with fractal method.This method of generating sky infrared image is applicable,and the results prove to be reliable.