Abstract: The mechanical property degradation rules of pipeline steel with damage have been established through tensile test of X70 steel specimens containing mechanical holes.Elastic modulus E and yielding strength σs decreased observably with hole ratio when.The hole ratio was up to 50%,E and σs degrades to zero.Based on damage theory,residual strength assessment formula has been developed by modifying API RP 579 formula for volumetric flaw evaluation.The results of case analysis calculation indicated that the conservation extent of evaluation result of pitting defect would increase with pitting corrosion depth if it was simplified as a volumetric defect.If the maximum depth of pitting corrosion is less them t/10,this kind of engineering simplification method can be accepted.