In this study, oxygen vacancy-rich magnesium oxides (SH-MgO and SC-MgO) were successfully prepared by the combination of chemical precipitation with high-temperature calcination. Effects of alkaline reagents (NaOH and Na2CO3) on surface morphology, pore structure and particle size distribution of SH-MgO and SC-MgO were analyzed. Besides, the phosphate removal performance and mechanisms were also investigated. The specific surface area of SH-MgO and SC-MgO was 98.03 and 62.35 m2 g−1, respectively. The corresponding average particle sizes were 211.98 and 19.56 μm. The phosphate adsorption followed the Vermeulen model and thus the rate of adsorption was limited by the intraparticle diffusion. The high affinity and selectivity of SH-MgO and SC-MgO for phosphate implied their great potentials in practical application. The EPR results demonstrated the presence of large numbers of oxygen vacancies in SH-MgO and SC-MgO. The phosphate removal mechanisms mainly included oxygen vacancy capture, electrostatic attraction, surface precipitation and inner-sphere complexation. This study provided a new strategy for facile preparation of oxygen vacancy-rich magnesium oxide, which was extremely significant for advanced removal of phosphate to reduce eutrophication.