Abstract: With a comparative analysis,the progress with R & D of the C125 casing for mildly sour environments as made abroad is generally described here in the paper. Also elaborated are the fit-for-purpose environment and the SSC evaluation test of the said casing. It is pointed out that the C110 grade is the highest one among the SSC-resistant steel grades,which evaluation result is made through the method A,the solution A as per NACE TM0177 up to the pH-P H 2 S plate region Ⅲ as per NACE MR0175/ISO 15156-2. Further described are the fit-for-purpose environment evaluation and their application to development of the C125 grade for mildly sour environments.