The MgO with different reactivity was prepared by being calcinated at 900℃ for different time. The fluidity, setting time, mechanical strength, pH value of the cementitious materials prepared by mixing silica fume(SF) and activated MgO were investigated. The effects of MgO with different reactivity on the hydrated products of MgO SF paste and hydrated magnesium silicate（M S H）structure were characterized by an X ray diffraction（XRD）, simultaneous thermal analysis（DSC TG DTG） and Fourier transform infrared spectroscopy（FTIR）. The results show that with the decreasing of calcination time, the intensity of MgO diffraction peak is declined, the peak width is widened, the grain size is refined, and the activity is increased. The MgO SF prepared by the MgO with highly reactivity has poorer fluidity of paste and mortar, shorter initial and final setting time, and higher strength. MgO SF prepared by MgO calcined at 900℃ for 20h possess the longest setting time and the highest compressive strength. The content of Mg(OH)2 in M S H is increased gradually with the curing age increase from 0 to 3d, the content of Mg(OH)2 in M S H increased slightly and the strength collapsed at 28d. With the increase of curing age, the M S H content and the degree of polymerization degree of the silicate network structure in M S H are increased.