Abstract: Elaborated here in the paper are the thinking concerning developing the ultra-high strength 125S SSCresistant casing by means of microstructure design and systematical full-process flow control.Also tested are the mechanical properties and SSC resistance of the casing,and analyzed are the microstructure characteristics of the casing.It is regarded via the analysis that the said casing possesses the mechanical properties as up to relevant requirements,and rather strong SSC resistance;the casing'microstructure features the weak oriented structure texture111<110> and011<111>,and a high-density dislocation with good mobility induced by phase transformation,as well as a lot of dispersed alloy carbide and ultra-fine lamellar pearlite,which from the perspective of the microstructure design,not only ensures the high strength of the 125 steel,but also provides high density hydrogen irreversible traps,and therefore greatly improves the SSC resistance of the casing.