Abstract: Since bearing capacity of a casing will reduce when the casing is worn,it is very important to predict or evaluate the collapse capacity of the worn casing.To combination numerical simulation and actual tests,the collapsing failure mechanism of worn casing and the factors and rules impacting the collapse strength were studied.Collapse of crescent worn casing belongs to "three hinged" instability.The severely-worn position on the casing was yielded into plastic zone first and deformed greatly,which resulted in the plastic instability of the whole structure.The collapsing strength of the worn casing presented changes of exponent,power function,and linear trend with the residual casing wall thickness,wear radius and axial load respectively.Based on this study,the calculation model of collapsing strength for the whole worn casing were established and the model was verified by tests,which showed the relative error between calculated from the model and those tested was within 5%.