Abstract: In order to solve the calibration of the infrared imager test equipment, a kind of infrared radiometer in which the temperature of interior reference blackbody was equal to the environmental temperature was designed. In order to evaluate this kind of radiometer and give guidance to the infrared radiometer design, a mathematic model for infrared radiometer noise equivalent temperature difference （NETD）was given. The NETD of infrared radiometer was estimated under different electronic bandwidths and spectral wavebands. The results show that when noise equivalent bandwidth is within 1 Hz, NETD of infrared radiometer in mid-wavelength infrared (MWIR) and long wavelength infrared (LWIR) spectral bands is smaller than 0.01℃, which can meet the requirements of the calibration of infrared imager test equipment.