Abstract: In this paper,the merits and shortcomings of the most representative ten kinds of the noise equivalent temperature difference(NETD) models in several decades are discussed, and the new prevalent NETD model is set up. The new NETD model eliminates the great f/# effect from the optical system. The effect of the duty ratio in the math model is taken into account and two options of spectrum filtering functions (Fλ1,Fλ2) are added. The new model which has been established by us can be well used in the performance evaluation of temperatur sensitivity of the thermal imaging system.