In order to study the effect of sal perlatum water glass on hydration behavior of the alkali activated slag cement(AAS), the setting time, slumping expansion degree with time and compressive strength of AAS were studied. Hydration behavior was measured by micro calorimetry. Regulation of hydration kinetics of AAS was evaluated. Hydration products and microstructure of AAS were analyzed by X ray diffraction(XRD) and scanning electron microscope(SEM). Mechanism of hydration behavior of AAS was discussed. The results show that when the amount of Na2O in sal perlatum is less than 20%(by mass), the setting time of AAS paste is longer, early hydration process of AAS is controlled by dense diffusion, the reaction rate of AAS increases with time. While the amount of Na2O in sal perlatum is more than 80%, the paste is quick setting. There is no Ca3(PO4)2 crystal in the early hydration production of AAS. Thus，Ca3(PO4)2 is not the factor to retard the setting of AAS.