Calcium sulpho-aluminate modified portland cement (SMP), containing3.0%, 5.5%, 7.0%, 9.0% of MgO were prepared.DTA, XRD, SEM and EDAX are used to investigate the effect of MgO on sintering of SMP.The results show that MgO can accelerate the decomposition rate of CaCO3 and lower the decomposition temperature about 10 ℃.The amount of periclase increases with the increase of MgO content.Periclase in the clinker exists mainly around the hole.Periclase generally growth with the intermediate phase, the size of the periclase is 1~4 μm.