Effects of slag synthetic strength-enhancer（SE）on the properties of cement-based materials were investigated in the paper. The mineral composition and chemical structure of the SE, as well as the hydration of cement, and its hydrates and microstructure were analyzed by microcalorimeter, XRD, DSC/TG, SEM and IR, and the strength-enhancing mechanism of the SE was discussed. The results show that the water requirement of normal consistency of cement is enhanced, the setting time of cement is shortened and the fluidity of cement paste is reduced by adding SE; the strength of cement-based materials at different ages (early, middle and later) are effectively enhanced by adding SE; the hydration reaction rate of cement is significantly accelerated and the amount of hydrates such as AFt and CSH in cement paste increases by adding SE, but the content of Ca(OH)2 is reduced; the SE contains lot of silica/aluminium hydrates or sol particles with high chemical activity, which can react with Ca(OH)2 to form more CSH, and therefore the hardened cement paste become denser and stronger.