Change in the quantity of bound chloride ions in the cement pastes, were investigated after them immersed in 5% Na2SO4 solution for 0,28,56,90 and 120 days. Meanwhile, the mechanism of the release of bound chloride ions due to Na2SO4 attack was discussed with the results from X ray diffraction(XRD), Fourier transform infrared(FTIR) and derivative thermogravimetric analysis(DTG). The results indicate that the stability of bound chloride ions is very susceptible to Na2SO4 attack, which lead to the transformation of chloride from bound state to free state. That Cl- in Friedels salt is substituted by sulfate ion from sodium sulfate solution is the main cause for the decomposition of Friedels salt. The monosulfoaluminate(AFm) formed during the ion exchange process between Cl- and SO2-4 would transform to ettringite(AFt), which could accelerate the formation of the secondary ettringite. The decomposition of Friedels salt due to Na2SO4 attack played domain role in the release of bound chloride ions. The stability of bound chloride ions absorbed by C S H gel is no susceptible to Na2SO4 attack.