Imaging Model in space domain and frequency domain of an infrared camera with focal plane detector is established. It shows that because of the existence of back ground radiance and non-uniformity the geometrical performance of an infrared camera can be evaluated by MTF only when the camera is well-corrected and the linearity and space invariance condition is satisfied. An MTF measurement method of an infrared camera using focal plane detector is given. This method does not depend on the well correction of the camera and can avoid the deviation from the background radiance.