Abstract: The yield strength and tensile strength of high-grade steel casings such as Q125,V140 and V150 were obtained by indoor experiments under high temperature conditions.The strength reduction factor was introduced and its equation applicable for the high-grade steel casings at high temperature was regressed from the experimental data.The strength reduction factors at 150℃ and 200℃ were given for convenient applications in oilfields,adopted by Chinese Petroleum and Natural Gas Industry Standards and applied to the design of high temperature casing strings.Design verification and safety evaluation of deep-well casing strings in western China based on the experimental results and the regression formulas show that the experimental data have good versatility to meet the requirements of field casing design.The yield strength and tensile strength of the high-grade steel casings decreases greatly with increasing temperature,and the decrease rate of yield strength is greater than that of tensile strength.It is suggested that the influence of temperature on the material properties of the high-grade steel casings should be considered during the design of high-temperature well casing strings.