Occurrence state of Mg2+ in clinker and hydrates was observed and analyzed with optical microscope, scanning electron microscopy(SEM), backscattered electron image(BSE) and energy dispersion X ray analysis(EDAX). The results show that when the growth space is sufficient, the periclase forms octahedron structure, and subhedral or anhedral crystal is formed in limited space in clinker. Grain size of periclase in clinker is mainly in the range of 100650μm, most of the grains size ranges from 250 to 300μm, and the length of the largest particle is up to 4000μm. At the later stage of cement hydration, the growth rate of Mg(OH)2 crystal along the c axis direction is accelerated and the particle thickness is increased notably. MgO dissolved in the clinker aluminum solid phase forms Mg6Al2CO3(OH)16·4H2O after in situ hydration and carbonization. Mg2+ introduced from clinker can form magnesium calcium silicate hydrate with Ca2+, Si4+, etc., or be absorbed on the surface of calcium silicate hydrate.