Abstract: To solve the detection distance of infrared thermal imager, using MODTRAN program, the atmospheric spectral transmittance in different cloud and rain conditions was calculated. For detector anglefixed and heightfixed operating, using the spectrumpath bisection method and spectrumangle bisection method respectively, the thermal imaging system detection distances of point source target were evaluated. Under cirrus weather conditions, the effects of the detection probability, rainfall rate, cloud thickness and cloud base height on the atmospheric spectral transmittance and the detection distance were analyzed. The results show that the selection of detection probability of infrared thermal imaging system has a great influence on the detection distance. In the case of tank target, when the detection probability increases from 10% to 90%, the detection distance reduces by 21.7%. The rainfall rate also has a great influence on atmospheric transmittance and the detection distance. When the rainfall rate increases from 0 to 4.0 mm/h, the detection distance decreases by 76.8%. The effects of the cloud thickness and cloud base height are associated with the geometric position relationship between the detector and the cloud. The upper bound of the detection distance of human target is 5.60 km. The detection distance is also influenced by the detector operating model. The biggest difference is about 3.8%.