Based on the ternary cementitious materials composed of cement, fly ash and slag, the mineral admixture metakaolin was added to prepare quaternary cementitious materials. The relationship between hydration products of quaternary cementitious materials and chloride binding capacity was qualitatively analyzed by X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG). The content of chloride by chemical binding and physical adsorption was quantitatively analyzed by TGA/DTG and Rietveld’s external standard method. The results show that the addition of metakaolin increases the hydration rate of cement in early stage, and promotes the hydration of fly ash and slag powder, which makes the quaternary cementitious material hydrated to produce more AFm (4CaO·Al2O3·CaSO4·13~19H2O) and C-S-H gel. At the same time, the quaternary hydration system also increases the molar ratio of aluminum to calcium, which makes the monosulfoaluminate more inclined to the conversion of monocarboaluminate in the presence of carbonate. The results of isothermal adsorption of chloride ions show that the content of AFm phase is positively correlated with the binding capacity of chloride ions. The results of Rietveld’s external standard method show that with the addition of metakaolin, the chemical chloride binding capacity of quaternary system is increased, but the physical adsorption capacity of quaternary system is reduced. Compared with the ternary system, the chemical chloride binding capacity increases by 94.16%, and the physical adsorption capacity decreases by 7.62%. TGA/DTG quantitative results show that Rietveld’s method is feasible.