This paper deals with the influences of the S/Al molar ratio of cement and the sulfate in extravagation solution on the chloride threshold value via the linear polarization method,the mercury injection test and XRD.The results show that,with the increase of S/Al molar ratio,the chloride threshold value first increases and then decreases,while the chloride bounded in cement hydration products keeps reducing,that the S/Al molar ratio corresponding to the maximum chloride threshold value decreases with the increase of Al₂O₃ content of cement.More-over,it is also found that,when the specimens are immersed in the composite solution with the SO₄^2- and Cl^-concentrations similar to those of seawater,the chloride threshold value first increases and then decreases with the increase of S/Al molar ratio,but it is lower than that of the specimens immersed in NaCl solution,and that,with the penetration of the mortar first becomes denser and then presents a volume expansion due to an excessive penetration,thus resulting in the decrease in chloride threshold value.