Absolute spectral responsivity is one of the important technical parameters of detectors. With the development of terahertz detection technology, it is becoming increasingly important to measure the absolute spectral responsivity of terahertz detectors accurately. Due to the lack of a continuously tunable terahertz light source and spectroscopic system in the terahertz band, the traditional method of measuring the spectral responsivity of ultraviolet, visible and infrared detectors can not be used to measure the spectral responsivity of terahertz detectors. In this paper, the relative spectral responsivity of 2~10 THz is measured based on the reflection method, and CO2 measures the absolute responsivity of 2.52 and 4.25 THz pumped gas laser as the light source, and the absolute spectral responsivity of 2~10 THz is obtained. The two frequency points of THz absolute responsivity and relative spectral responsivity are mutually verified. The ratio of absolute responsivity measurements at 2.52 and 4.25 THz is 0.753, and the ratio of relative spectral response measurements is 0.749. The difference between the two is only 0.004. Therefore, the reflection method used in the article to measure the relative spectral response of the terahertz detector is feasible. In addition, the test of water vapor in the terahertz band has a great influence. This article tests the attenuation characteristics of the atmosphere in the 1.5~10 THz band. The test shows that water vapor has a significant attenuation effect on the terahertz wave. When measuring different environmental humidity Will produce different results, so the humidity of the atmosphere needs to be strictly controlled during the measurement process of the terahertz detector. Especially before the 3.3 THz band, due to its weak signal, if the water vapor is too large or changes greatly during the test, the testing effect will be seriously affected. The system can satisfy terahertz detectors’ development, production, detection and application. It can guide material selection, process improvement, data compensation, optical system design and image processing, and promote the effectiveness of terahertz weapons and equipment. Therefore, the measurement of the absolute spectral response of the terahertz detector is very important for device designers, imaging equipment system designers and device users.