It is hard to calculate the operating distance of infrared system accurately in real time in the complex atmospheric conditions, so we proposed to build a realtime calculating system. MODTRAN software was combined with the operating distance model based on spectral bisection method of infrared imaging system. This system realized automatically written configurations for MODRTRAN software to simplify the configuration method of atmospheric model. Configuration file generated from MODTRAN was automatically read.The infrared system operating range of a target was calculated at different altitudes (1 km~6 km) and with different rainfall intensity (0 mm/h~4 mm/h) .The results show that the variation of the measuring height has obvious influence on the infrared system operating distance, and the influence is great at 1 km~3 km; the rainfall has huge impact on the infrared system operating distance, the operating distance attenuates to half of that with no rainfall when the rainfall intensity is 1mm/h, and the operating distance tends to stable when the rainfall intensity is more than 3mm/h.