The inadequacies of traditional noise equivalent temperature difference (NETD) test are pointed out; the commonly used NETD test formula is derivatived from the principle of NETD test. The accurate measurement methods under laboratory conditions are raised and test results are corrected; NETD of cooled HgCdTe infrared push-broom system with different bands, different optical lens are tested and four sets of data on the test are got and contrast analysis is given to them. According to the detector temperature response characteristics and blackbody radiation characteristics of the medium wave and long wave band, the reasons for NETD test result differences under different test conditions are explained theoretically. At the same time, experimental result shows that NETD with the same band and the different optical calibers change little, while NETD with different bands and the same optical caliber change very much. The experimental results also confirm that only the F number in all the parameters of the optical system, is relative to NETD.