In this paper, the MgO with different reactivity was prepared by the secondary constant temperature calcination of the raw material light burned magnesium oxide powder for 1.5 h at different temperatures. The setting time, fluidity, compressive strength and reaction solution pH value of a new type of magnesium silicate hydrate cement cementitious materials (also known as magnesium silicate phosphate hydrate cement, MSPHC) prepared with different reactivity MgO, silica fume (SF) and dipotassium hydrogen phosphate (K2HPO4) were investigated. The mechanism was analyzed through X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG) and scanning electron microscopy (SEM). The results show that with the increase of calcination temperature, the intensity of MgO diffraction peak increases and the reactivity decreases. The faster the setting time and poorer the fluidity of the MSPHC purified slurry prepared by MgO with higher reactivity, while the MSPHC prepared with moderate reactivity MgO can achieve better mechanical properties. The most important hydration product of MSPHC is magnesium silicate hydrate (M-S-H) gel, besides, Mg(OH)2 and MgKPO4·6H2O (MKP) are also generated. The MgCO3 component in the raw material light burned magnesium oxide powder does not participate in the system reaction. MSPHC prepared by MgO with moderate activity has the highest hydration product M-S-H gel generation within 28 d of age, so the hardened body has the highest compressive strength. The higher the reactivity of MgO in the MSPHC reaction system is, the faster the dissolution rate is, the faster the hydration reaction process is.