It is found that the incorporation of Nitrite Corrosion Inhibitor (NCI) greatly weakens the resistance of mixtures to sulfate attack.To study the mechanism of this phenomenon,in this paper,the influence of NCI additionon on the cement paste and microstructure change of high performance concrete specimens is studied by means of quantitative XRD,SEM tests.The results demonstrate that the incorporation of NCI accelerates the formation of calcium hydroxide and ettringite crystals,and weakens the pore refinement effect caused by the secondary hydration reaction of fly ash and microsilica.At the age up to one year,the relative crystal quantity in mixture containing NCI is always higher than that in control mixture.The reasons for the degradation in sulfate resisitance of mixtures may be attributed to the increase and stability of the calcium hydroxide and ettringite crystals formed and the weakening of secondary hydration reaction.Based on the results,conclusion can be drawn that NCI should be used cautiously in practical engineering where high resistance to sulfate attack is required.