Carrying capacity of the casing will reduce after the casing is worn,which exerts the significant impact on the subsequent well drilling,well completion,oil extraction and well repair.With the combination of theoretical analysis,numerical simulation and actual test,the collapsing failure mechanism of casing wear and the factors and rules impacting the collapse strength are studied.Collapse of crescent casing wear belongs to "three hinged" instability.The severely-worn position on the casing is yielded into plastic zone first and deformed greatly,which results in the plastic instability of the whole structure.The collapse strength of the casing wear presents changes of exponent,power function,and linear trend with the residual casing wall thickness,wear radius and axial load respectively.Based on this,the computation model of collapsing strength for the whole casing wear has been established and the model is tested for verification by computing collapse strength of non-casing wear.The study results provide a guide for design of the casing sting strength and down-hole safety.