The study of infrared radiation properties of military targets is important for the evaluation of IR detection range. The limitation and defect of the IR property inversion method based on infrared images when it is used to inverse the properties of point targets are analyzed. According to the principle of radiation measurement, the mathematical relationships among the radiance, pixel grey value and radiation intensity are deduced by extracting the point target in an IR image. A new algorithm for inversing the IR radiation properties of a helicopter in point target detection is proposed and is used to inverse the infrared radiation intensity of a certain military helicopter. The radiation intensity distribution of the helicopter at different observational angles in the waveband of 3 to 5 μm is calculated. The results show that the algorithm can implement the inversion of IR radiation properties of small targets in far distance. It is of significance to the evaluation of infrared radiation detection range of IR reconnaissance systems.