The work aims to establish a set of corrosion environment analysis method for intermediate casing in HTHP (high temperature and high pressure) and high H2S gas well to determine the adaptability and safety period of casing material in extreme working conditions of drilling and gas production based on corrosion evaluation and experiment. Two extreme conditions of “all blow-out” in drilling stage and “gas breakthrough of production casing-intermediate casing annular” in production stage were analyzed based on the HTHP and high H2S gas field in the Southeast Asia offshore (reservoir pressure at 57.3 MPa, temperature at 155 ℃, H2S mass fraction at 0.46%, CO2 mass fraction at 6.44% and Cl? concentration of formation water at 1.6% to design the experiment conditions of corrosion materials. The adaptability of T95 casing in extreme conditions was evaluated through SSC (sulfide stress cracking) experiment and uniform corrosion mass loss test. If blowout condition occurred, the T95 casing could meet the 30-day treatment requirement and complex conditions, as well as the needs of following works. If annular gas breakthrough in production casing-intermediate casing occurred in production stage, the uniform corrosion rate of T95 casing at casing shoe was 0.3047 mm/a and the uniform corrosion rate of T95 casing near the wellhead was 0.7536 mm/a, thus providing a 5-year service life. Through the site operation, T95 casing could meet the requirement of complicate condition operation during drilling and production stages. Blow out in drilling stage and gas breakthrough in production stage are extreme conditions required to be considered in the design and implementation of HTHP and high H2S gas well. The adaptability and limit of intermediate casing can be tested according to the corrosion experiment with corresponding conditions to ensure the safety of on-site operation.