This study systematically investigated the impact of different calcination temperatures and holding times on MgO reactivity, as well as the influence of varying reactivity and lattice distortion of MgO on the properties of magnesium oxysulfate cement (MOSC). Results indicated that at temperatures of 700–900℃, magnesite underwent decomposition, resulting in increased lattice distortion, and heightened reactivity. At temperatures of 1000–1100℃, magnesia began sintering, nucleating, and experienced reduced crystal distortion, leading to decreased reactivity. Notably, the appearance of the pseudomorphic mother rock at calcination temperatures of 900℃ and 1000℃ was associated with decreased magnesia reactivity. Moreover, the compressive strength of MOSC prepared using MgO calcined at 900℃ for 6 h increased to 77 MPa after 28 d of air curing. The hydration reactivity and lattice distortion of MgO could be regulated through changing the calcining temperature and holding time, which provided a theoretical basis for the preparation of magnesium oxysulfate cement with excellent mechanical properties.