Operating distance of Rosette scan infrared detection system is hard to be estimated by infrared detector operating distance formula in complex background. So correlated to the operating distance model of Rosette scan infrared detection system, the model working in complex background was developed. Using the algorithms that calculates the Rosette scan infrared detection system responses pluses characteristics via gray scale of background collected by infrared thermal imager and verified by field tests, the result shows that the improved model can estimate the operating distance of Rosette scan infrared detection system under complex background more efficiently.