For the requirements of NETD measurement in engineering,an automatically testing method and system for NETD are designed. This system uses off-axis aspheric two-double light route. A collimator with 120 mm aperture and 720 mm focus is designed,which can reduce the volume of system. Black body uses TEC as source for heating and refrigerating. Temperature control precision reaches 0.05 ℃.Testing time is only 5 s by improving algorithm,which can improve the detection efficiency. Compared with CI system,the consistency and precision of NETD measurement are good. This system has an application value in engineering.