Metallic phosphides as a crucial class of metal-like compounds show high electric conductivity and electrochemical properties. It is of significant benefit to understanding the relationship between the electrocatalytic performance and phosphating degree of precursors. In this work, using Co3O4@SiO2 as precursor, core-shell structured CoP@SiO2 nanoreactors with outstanding oxygen evolution reaction performance were synthesized through a facile calcination method. The electrocatalytic performance of CoP@SiO2 modified electrode that treated with 500 mg NaH2PO2 was greatly enhanced. The obtained product displays a low overpotential of 280 mV at a current density of 10 mA/cm2 and a Tafel value 89 mV/dec in alkaline conditions. The easy available CoP@SiO2 with outstanding catalytic performance and stability possesses huge potential in future electrochemical applications.