Abstract: Owing to the fact that the alloy element contents in the ferrite phase and the austenitic phase of the dual phase stainless steel are different,the pitting resistance equivalent number PREN between the two phases are different,which in turn leads to difference of corrosion resistance between the two phases,thus easily causes development of local corrosion.The mechanism of SSCof the dual-phase stainless steel seamless oil tubing and casing is studied.And the result shows that at the ambient temperature due to the joint effect by H₂S and tensile stresses,it is austenitic pitting that causes the failure of the 2205 dual-phase stainless steel,and after subjected to the SSC test as made at 90℃,the said steel has both austenitic pitting and ferrite selective corrosion occurring at the same time.