SiO is regarded as one of the most promising anode materials for next-generation lithium-ion batteries due to its high energy density and low cost. However, practical applications of SiO are seriously limited by its low initial Coulombic efficiency. Herein, we report an affordable manufacturing method to synthesize carbon-coated CSiOMgSiO3 Si composites with a secondary superparticle structure and an outstanding initial Coulombic efficiency. The in situ evolution of the MgSiO3 phase through the reaction between MgO and SiO2 contributes to the improved initial Coulombic efficiency. The formation of MgSiO3 consumes the deleterious SiO2 phase within SiO, and MgSiO3 acts as an electrochemically inactive phase that can effectively buffer volume variations of active materials. These attributes and the unique mesoporous superparticle structure lead to a markedly improved electrochemical performance. The resultant CSiOMgSiO3 Si anode exhibits an initial Coulombic efficiency of 78.3%, which, to our knowledge, is the highest value ever reported for SiO-based anodes without prelithiation. This work offers a new strategy for addressing the intrinsic low initial Coulombic efficiency of SiO, paving the way toward the mass production of SiO-based anodes for real applications.