By analyzing the characteristics of fracture signal during deep well injection in Dongying depression of Shengli oil field, comparing with the induced earthquake at Jiao 7 well in the same depression, the seismic mechanism of earthquake induced by water injection in oil and gas field is discussed. It influenced greatly on induced earthquake whether there are existed fracture or not. It is considered that the magnitude of induced earthquake is closely related to the ratio of injection and production, and the structure location of injection well.