The environment in which an aerial infrared camera operates is studied and the fact that when the camera operates at high altitude, the variation of temperature and atmospheric pressure can cause the focus of the camera to be changed and hence affect the imaging quality is pointed out. The effect of temperature and atmospheric pressure on the imaging quality of the camera is summarized. According to the features of the operation environment of the camera, the rules of temperature and atmospheric pressure causing the camera to be defocusing are analyzed respectively. An approximate calculation method is proposed and is verified in test. For different cameras, the corresponding convergence compensation methods are given. This work is of a certain reference value to the development of infrared cameras.