Due to materials, process and use environment, bad pixels exist in the focal plane of infrared thermal imager. Firstly, according to the analysis of infrared image characteristics of bad pixels and using collected IR picture, the pixels of the focal plane are detected one by one with a 5×5 window. Secondly, the bad pixel detection criterion is obtained through calculating the bad pixel membership degree based on the change of gray level and direction. Finally, the validity of the method is proved by an example.