During a large earthquake's gestating and occurring process, the stress-strain state along the fault zone has the close relation with the weak seismicity around the fault zone. The seismicity's energy release near the fault zone structure before a earthquake can reflect better the dynamic process of a earthquake's gestating. Thus, in this paper, the method of natural perpendicular function development has been adopted to discuss the time variation about the energy field of the seismic activity along the Longmenshan fault zone before the Wenchuan Ms8. 0 earthquake, 2008. The results have showed that, before the Wenchuan Ms8.0 earthquake, the time factors of the typical field corresponding to several key eigenvalues of the energy field along the Longmenshan fault zone demonstrated a very clear short-term rising, probably as the short- term anomaly message for this earthquake. By analyzing contrastively earthquake examples such as Tangshan earthquake of 1976, the authors believe that studying the time variation of energy field of the seismic activity along the active fault zone can help conduct the intentional and intensive monitor and forecast on the active fault region where is relatively dangerous based on earthquake med- and long-term decision.