Effects of slag synthetic strength enhancer（SE） on the properties of cement based materials were investigated. 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, X ray diffraction(XRD), differential scanning calorimetry thermogravimetric analysis(DSC TG), scanning electron microscope(SEM) and infrared spectra analysis（IR）, and the strength enhancing mechanism of the SE was discussed. The results show that the addition of SE enhances the water consumption of cement standard consistency, shortens the setting time of cement and reduces the fluidity of cement paste. SE has good strengthening effect on the strength of cement based materials at early, middle and later ages. SE can significantly accelerate the hydration reaction rate of cement, and increase the amount of hydration production such as ettringite(AFt) and calcium silicate hydrate（C S H） in cement paste. but reduce the content of Ca(OH)2. SE contains lots of silica/alumina hydrates or high activated sol, which can react with Ca(OH)2 to form more C S H hydration products, thus cement hardening paste may be more compact and stronger.