In the 87Rb- 129Xe spin exchange system, the accurate measurement of the relaxation times of 87Rb and 129Xe is crucial for applications related to nuclear magnetic resonance gyroscopes and magnetometers. In response to this requirement, the influences of pump light and excitation magnetic field on 87Rbrelaxation time, as well as the spin exchange processes of 87Rb and 129Xe in the dark state, are analyzed. Based on theoretical analysis, a dark state sweep frequency measurement method is proposed. The measurement of longitudinal relaxation time of 87Rb and 129Xe is achieved. The transverse relaxation time and longitudinal relaxation time of 87Rb measured using this method are 1. 36 and 5. 18 ms, respectively. The longitudinal relaxation time of 129Xe is 519 s, whose R-squared is 0. 999 9 which has a very high goodness of fit. Compared with previous measurement methods, the dark state sweep method can completely eliminate the influence of magnetic field gradient caused by pump light, and has the advantages of high accuracy and easy operation. This article has high reference value for the performance analysis and calibration of nuclear magnetic resonance gyroscopes and magnetometers.