The long-time trend change shape and evolutionary characteristics of subsurface fluid in North China before MS≥6.0 earthquakes since 1976 are analysed.The results show that the begin time of long-time trend change of subsurface fluid is earlier,and the range is wider,with the characteristics of compatibility,repetition and migration etc.The migrate direction along major tectonic belt is from wide outer regions to epicenter area,or from epicenter area to wide outer region.The shape of changes show trend turn change,damage annual variation and accelerating variation etc,among them the trend turn change is majority shape.Study on level of underground water shows that the majority variation shapes of long-time trend change on level of underground water are slow elevation or descending diminution.Then long-time trend change of subsurface fluid and crustal deformation before strong earthquakes are comparatively studied.The results discover that the begin time of crustal deformation is earlier than it of subsurface fluid,the tensor-pressure variation of crustal deformation is more closely related with the level variation of underground water,namely if change of crustal deformation shows mostly slow pressure or tensor diminution,the change of underground water level shows slow elevation or descending diminution.Finally,the mechanism on long-time trend change of subsurface fluid before strong earthquake is discussed.Slow incremental tectonic stress make crack of rock closure and crust deform,the crust deformation changes porosity,pore pressure and permeability,causes the long-time trend abnormal change of subsurface fluid in the early stage of preparation earthquake.The tectonic stress slowly enhancing and congregating to future seismic center cause the change to migrate into future seismic center.The continuously upwelling of deep material and increment influencing range make the change migrate from future seismic center to outlying zone.