Basic magnesium sulfate cement （BMSC） with different molar ratios was prepared from different active MgO decomposed by calcination of magnesium hydroxide. The development law of compressive strength was analyzed. The influence mechanism was analyzed by hydration exothermic， liquid conductivity， X-ray diffraction （XRD），scanning electron microscope （SEM） and mercury intrusion porosimetry （MIP）. The results show that when molar ratio （n（MgO）/n（MgSO₄）） is 5， the strength increases first and then decreases with the increase of MgO activity in the first day， resulting from the fact that the higher MgO activity yields the faster hydration， on the contrary， too high MgO activity is not conducive to the formation of hydration strength phase （5·1·7 phase）. With the complete hydration of MgO， the strength increases with the decrease of MgO activity in the later days due to the 5·1·7 phase in BMSC prepared with low MgO activity The degree of crystallization is high. When the molar ratio is 7， the third exothermic peak of Mg（OH）₂ nucleation and growth appeared in the hydration process of BMSC， which led to the strength of BMSC to shrink. The main pores in high strength BMSC are intergranular pores， and the higher the crystallinity of hydration products， the larger the pore size.