Abstract: In drilling deep and ultra-deep wells, such issues like casing worn area, worn depth, remaining collapse strength and burst strength under uniform loading conditions are of widely concern. By theoretical analysis and finite element method, a set of formulas on the calculation of those values are presented. Meanwhile, a formula for casing remaining collapse strength under non-uniform loading conditions is also established by converting a non-uniform loading into an equivalent uniform loading. The study shows theoretical results agree well with published test datum. Casing wear and formation elastic parameters should be considered in the casing strings design and strength assessment in an ultra-deep well drilling. At the same time, more experiments on worn casing strength evaluation are suggested to conduct for different types of casing to verify applications of the developed theoretical formulas.