Abstract: Deformation or offset of well casing is an event induced by water injection, similar to induced earthquake in many aspects. Observations in Fuyu Oil Field indicate that the deformation of casing has close relationship with water injection, especially with injection pressure. The number of casing daformation increases as the injection pressure rises. The events happen mostly in the bottom part of Qinshankou mudstone (b₁) with developed bedding and nearly vertical fissures. The orudstone soaked with injected water is weakened in strength. Besides deformation in casing cross section, relative displacement in axial direction also happens, which indicates the result of shearing. A possible mechanism of slip along bedding in the orudstone is praposed in this paper to interpret the observations. The ratio of shear stress component to normal stress component on the bedding is too small as compared with frictional coefficient,therefore, if no water is injected or the injection pressure is not high enough, no slipping of rock mass occurs and no casing deformation consequently. A critical pore pressure of 66-76 bars is suggested to be the pore pressure required to initiate slipping and to cause considerable casing deformation.