Biodegradable polymer microspheres that can be used as drug carriers are of great importance in biomedical applications, however, there are still challenges in controllable preparation of microsphere surface morphology and improvement of bioactivity. In this paper, firstly, poly(L-lactic acid) (PLLA) was synthesised by ring-opening polymerisation under anhydrous anaerobic conditions and further combined with the emulsion method, biodegradable PLLA microspheres (PM) with sizes ranging from 60-100 μm and with good sphericity were prepared. In addition, to further improve the surface morphology of PLLA microspheres and enhance their bioactivity, functionalised porous PLLA microspheres loaded with magnesium oxide (MgO)/magnesium carbonate (MgCO) (PMg) were also prepared by the emulsion method. The results showed that the loading of MgO/MgCO resulted in the formation of a porous structure on the surface of the microspheres (PMg) and the dissolved Mg could be released slowly during the degradation of microspheres. In vitro cellular experiments demonstrated the good biocompatibility of PM and PMg, while the released Mg further enhanced the anti-inflammatory effect and osteogenic activity of PMg. Functionalised PMg not only show promise for controlled preparation of drug carriers, but also have translational potential for bone regeneration.