In this study，“active source” refers to large-volume airgun controllable sources. China has successively established four active source excitation platforms in regions including Yunnan， Xinjiang， and Gansu， through which long-term continuous observation data have been obtained. Monitoring of multiple strong earthquake preparation processes has revealed significant anomalous changes in subsurface medium wave velocity prior to earthquakes， with seismic phase travel times exhibiting a characteristic “V”-shaped variation. To reveal the physical mechanism of this phenomenon， this study employs a rock crack wave velocity model to divide the stress evolution process in rocks into Stage Ⅰ and Stage Ⅱ and investigates the dynamic variation patterns of rock compressibility factor， fragmentation degree factor， wave velocity， stress， and crack development in these two stages. Using the mathematical formulas of this model， the actual variation trends of medium wave velocity and seismic phase travel time in both stages were calculated. The calculation results show that the theoretical outcomes are not only highly consistent with rock mechanics theory and experimental data but also align with the “V”-shaped travel-time variation characteristics observed by active sources. This research （i） provides theoretical support for the quantitative interpretation of precursory wave velocity anomalies before strong earthquakes and （ii） enhances the understanding of the mechanisms behind subsurface medium changes prior to earthquakes， thus holding significant scientific importance for mediumto short-term earthquake prediction.