In recent years, broadband seismographs have shown the obvious superposition of abnormal seismic observation signals a few days prior to large earthquakes. These signals can be distinguished in the frequency domain by frequency spectrum analysis. Study results show that broadband digital signals contain rich information related to crustal movement and damage. In this study, based on the fatigue damage theory, we computed and analyzed the broadband digital signals from the ENH, KMI, and LSA stations three days prior to the 2008 Wenchuan MS8.0 earthquake. The results show that the relative equivalent stress indicated by the signals increased significantly in the three days prior to the earthquake.