Abstract: With the exploitation of high sulfur gas fields, the working environment of tubing and casing gets worse and more complicated. Under sour environment, the yield failure criterion of casing is no longer applicable , Environmental cracking (EC) is the main failure mode. Even if the safety factor meets the yield criterion, safety of the structure cannot still be ensured. Therefore, the tubing and casing’s Fitness-for-Service assessment method, which is based on mechanical evaluation of environmental cracking, is proposed in this paper. This method is design safety factor of string strength on the basis of threshold stress percentage of environmental cracking. The criterion of this evaluation method is: in working condition, if the threshold stress of environmental cracking of tubing and casing is no less than the percentage corresponding to the required yield strength of tubing and casing, tubing and casing would run safely. Test evaluation method of threshold stress of environmental cracking usually adopts NACE method A, refers to the NACE Standard Tensile Test. This test and its relevant data handling are introduced in this paper. At the same time, the foreign relevant criteria on threshold stress of environmental cracking are studied with this assessment method.