Cloud is an important factor for the difficulty in air target detection and recognition. Based on the IR radiation of target and cloud, the effects of cloud on the target IR detection were analyzed. Firstly, according to the 4 different position relationships of target and cloud, the models for calculating the radiation difference of target and cloud that received by the detector were established, and then the calculations of the parameters in the model were discussed. Lastly, the simulation calculation was done as an example based on the model, and the results were analyzed. The results show that：in the band of 8-14 μm, the radiation difference of target and cloud has bigger value near the wavelength of 9 μm and direction angle of 50°, and smaller value appears at the longer wavelength and larger direction angle. The conclusion can provide the basis for the design of IR detection system.