The space environment of the three-axis stabilized satellite in geosynchronous orbit is complex, and the background radiation of the instrument varies greatly. The traditional stray light analysis method can not simulate the non-uniform temperature field, and can not calculate the instrument background in real time. Also the simulation error is large. Thermal radiation stray light integration method was used to analyze instrument background radiation of in-orbit infrared camera. The background radiation of the instrument and the signal-to-clutter ratio of the camera on the detector was calculated by using the real-time temperature fields with temperature gradient and radiation transfer factor. Comparing the result of the thermal radiation stray light integration method and traditional stray light analysis method with the on-orbit measurement, the error of the thermal radiation stray light integration method was less than 17%, while the error of traditional stray light analysis method was up to 114%. The result shows that the thermal radiation stray light integration method is closer to the actual on-orbit situation, and the simulation efficiency and accuracy are higher.