Magnesium oxide (MgO) is known for its bioactivity and osteoconductivity when incorporated into biodegradable poly(lactic acid) (PLA), whereas the weak interfacial bonding between MgO microspheres (mMPs) and PLA often leads to suboptimal composite properties with uncontrollable functionality. Conjugation of mMPs with PLA may offer a good way to enhance their compatibility. In this study, we systematically investigated two grafting techniques, solution grafting (Sol) and melt grafting (Mel), to decorate poly (D-lactic acid) (PDLA) on mMPs pre-treated by prioritized hydration to obtain Sol MPs and Mel MPs, in order to optimize the grafting efficiency and improve their controllability in the properties including the crystal structure and surface morphology. Meanwhile, the Sol method showed an improved grafting ratio (2.9 times higher) compared to the Mel method. The conjugation of mMPs with PDLA effectively neutralized the rapid pH increase during the degradation of pure mMPs, which could be used for sustainable delivery of the Mg