Currently there are a variety of functions for operating range calculation of infrared systems, each has its own applicability and limitations. Analysis was made to the existing operating range function based on contrast ratio, which showed that the effect of diffusion imaging and path radiation were not taken into consideration in the function, and it can only be applied to the target with a single wavelength. In this paper, the impact of diffusion imaging and the path radiation on the operating range of infrared systems were taken into consideration, the operating function based on contrast was modified, and a contrast-based function of infrared system's operating range to point targets was obtained. The forecasting result of this method was consistent well with the actual experimental result, which proved the validity of the method.