It is well known that there exists an optimal angle magnification which can make the thermal imaging systems and human eye matching optimally, thus, the thermal imaging systems attain the optimal performance. Anew optimal performance evaluation method was proposed to find the optimal angle magnification on the basis of SNR model. Based on MRTD channel width, a new evaluation parameter was presented for thermal imaging systems performance testing and evaluating. The optimal angle magnification or the optimal viewing distance can be acquired by using the new evaluation parameter through the experimental analysis for the thermal image systems. The new theory is of great significance for developing the evaluation theory of photoelectronic imaging systems, and it provides potential theory guidance for the design of practice systems.