Using air gun excitation signal data recorded by the active source observation system in the Qilian Mountain, travel-time change data of the P-wave phases before the Zhangye M5.0 earthquake on September 16, 2019 were obtained at each station of the observation system. Filtering, stacking, deconvolution, cross-correlation, and other data processing were performed. Clear velocity variations were found at the stations near the focal area before the earthquake. These results showed that the stress state of the underground structures in the focal area had changed in the months leading up to the earthquake.