Abstract: Long-time observed data of Subsurface fluid from a wide circumferential region of the Wenchuan M_S8.0 epicenter is analyzed before the earthquake. The result show that the subsurface fluids have long-time trend changes before the Wenchuan M_S8.0 earthquake. The changes are characterized by earlier emerging time, wider range, compatibility and migration toward the epicenter etc. The shape of subsurface fluid long-time trend change is characterized by trend turn, annual variation damage and accelerating variation etc, mostly trend turns. Further comparative study with crustal deformation shows that the long-time trend change migration of subsurface fluid and crustal deformation is identical, and the long-time trend change time of crustal deformation is earlier than that of subsurface fluid. The tensor-pressure variation of crustal deformation is more closely related with the level variation of underground water, namely if the long-time trend change of crustal deformation is the majority variation shape of slow pressure or tensor diminution, then the long-time trend change shape of of underground water level is the majority variation shape of slow elevation or descending diminution. Finally, the productive mechanism on long-time trend change of subsurface fluid is discussed before Wenchuan M_S8.0 earthquake .

Key words: Subsurface fluid, Crustal deformation, Wenchuan M_S8.0 earthquake, Long-time trend change mechanism