Abstract: The small parameter perturbation method was adopted to develop the stability criterion of a worn casing with initial ovality under non-uniform loadings, by comparing the relative strain of the worn casing against its critical value. The relationships between the relative strain and key parameters such as the outer diameter, the wall thickness, the initial ovality, the non-uniform loadings, and the worn depth of the casing were analyzed. The increase in the outer diameter of the casing decreased the critical value of the relative strain, and required a heavy casing to maintain its stability. The relative strain of the worn casing increased linearly with the initial ovality under non-uniform loadings. The relative strain was parabolically related to the non-uniformity of the loading. The bigger the non-uniformity coefficient, the greater the relative strain. The relative strain of the worn casing increased linearly with its worn depth under non-uniform loadings. The proposed stability criterion is expected to evaluate the integrity of wellbores which include worn casings subject to non-uniform loadings.