Abstract: The detection of point target is always an important research subject of infrared imaging system. Based on the irradiation distribution function of point target under action of point spread function, considering the positional relationship of irradiation distribution and unit detector,according to photoelectric conversion relation,the transformational relation of optical signal to electrical signal of point source detected by IRFPA was established. then, combining with detector noise model, new SNR model of point target was showed. There was a conclusion that detective point target SNR was closely related to point spread function, size of unit detector and line-of-sight displacement. This method could be a fundamental of accurate estimate of view range of infrared imaging system.