Abstract: It is impossible to evaluate the safety of the casing string after the yield strength point just by means of the conventional method for stress design and check of the casing string that adopts the yield strength of the casing as the design index. Addressing the fact that some damages of the casing are caused by rather high thermal stress during the ultra-heavy oil thermal recovering process，taking the TP110H steel as an example，a stress-strain double index design method is established to form a judgment criteria with the total strain less than the allowable strain，which not only meets the stress check of the string，but also breaks through the use limit of the casing strain，changing from elasticity to elastoplasticity. It is found via relevant research that the casing material exhibits creep effect and stress relaxation effect at high temperature，which greatly affects the thermal stress of the string during gas injection，well plugging and oil production. Since the creep strain is an important part of the total strain，the tensile stress in the oil production process can be effectively evaluated by means of integrating stress relaxation into the analytical calculation.