Abstract: Casing collapse accounts one of the main failures during production, therefore it’s of great importance to understand the remaining collapsing strength of worn casing. Suppose the worn part to be a circular arc, and could be looked as a plane model, the paper tries to analyze the collapsing strength from the two aspects of plane stress and plane strain, both applying the 8-node-element. The results show that collapsing strength decreases with the aggravation of casing wear and shortening of radius on both aspects, while the collapsing strength obtained by plane strain is higher than that of plane stress, which is lower than the minimum value of API standard. To further estimate the accuracy of this method, a spatial analysis method is adopted, which takes the worn part as cylinder with two spheres or ellipsoids, and applies the 20-node-element. The results show that the collapsing strength varies with the specific lengths of worn part and unworn part, but with a maximum and minimum value, when there is relative long worn part, approximately results are obtained by the two analysis methods.