With the development of terahertz detection technology, it becomes more and more important to accurately measure the relative spectral response of terahertz detector. The measuring principle of the relative spectral response of terahertz detector is analyzed, and a relative spectral response measurement system of terahertz detector is set up, the uncertainty of the measurement system is verified by using terahertz detector. According to the analysis of experimental data, the uncertainty of measurement is 9.2% in the range of 1 THz~10 THz, which can meet the requirements of relative spectral response measurement of terahertz detectors.