Abstract: In order to research the impact of nature environment on the synthesized performance of thermal imaging systems, a nature background noise factor was presented and introduced into MRTD-CW model comparing with Target Task Performance (TTP) model. An information quantity model based on MRTD-CW model with the nature background factor was proposed to evaluate the impact of nature environment on the synthesized performance of thermal imaging systems. The algorithm of the nature background noise factor was defined by experiments and the normalized form of the information quantity model with the nature noise factor was given. Taking typical scenes as examples, the impact of nature background noise on the synthesized performance of thermal imaging systems was analyzed by the information quantity model with nature background noise factor, and which was consistent with those of TTP model. Results show that the information quantity model with nature background noise factor can analyze the impact of nature background complexity on the synthesized performance of thermal imaging systems and can be applied to the evaluation of thermal imaging systems.