| Based on the actual condition of concrete sulfate attack in metro shield tunnel, the sagging moment mechanical model of longitudinal joint is established, which can take into account the details of joint structure, surface concrete and bolts. Furthermore the joint section can be discretized into different compression zones. Because the constitutive calculation of concrete is carried out in full integral form, it is adapted to analyze the crush of concrete, the yield of bolt and the limit state of longitudinal joint. Comparing with numerical calculation, the results of the two methods are basically the same, which verifies the rationality of the mechanical model. The results show that under sagging moment, the effect of sulfate attack on the mechanical property of longitudinal joint is mainly manifested after the contact of concrete at the outer surface edge. Against the increasing of sulfate attack time, the ultimate bending moment of longitudinal joint decreases continuously with an average margin. Meanwhile, the maximum deformation decreases against the increasing of attack time with a different decreasing margin. When the sulfate attack thickness increases, the ultimate bending moment and the maximum deformation of longitudinal joint decrease continuously with a larger margin.