The detection range of the infrared system is one of the key indexes of the photoelectric system, which determines the overall performance of the system.It is practical significance to evaluate the detection range of infrared system under laboratory conditions, which is not limited by weather or site.In this paper, starting from the relationship between the output signal of the detector and the target temperature of the blackbody, the expression of NETD is derived, the little Hudson formula is expressed by NETD, and the correctness of the formula is verified. The error between the index derived by this method and the theoretical calculation index is within 10%, which is feasible in practice.