Abstract: Casing shear damage appeared frequently in the datum bed in the Daqing Oilfield, and an uncementing technology has been adopted to delay casing damage. However, the critical value of marker casing shear damage is related to wall thickness and steel grade of the casing. An analysis was conducted on the elastoplastic mechanical characteristics of casings and formations, and a three-dimensional finite element calculation method was proposed, based on the engineering conditions of uncementing markers, for integral formation movement and shearing casing by using the Lagrange finite element equation, which takes account of the stress equilibrium state after large deformation. Critical values of the casing failure with different wall thickness and steel grade were calculated and compared at two failure conditions, i.e. the casing deformation and casing fracturing. It is shown that the critical slippage of casing with formation shear when the casing was fracturing is much smaller than that of casing deformation. The critical slippage of casing that have the same size and steel grade decreases linearly with the increase of wall thickness, but increases linearly with the increase of elongation rate. To sum up, the casing with small wall thickness and low steel grade should be adopted in datum bed of the Daqing Oilfield to delay casing damage on the condition that strength checks are satisfied.