Magnesium (Mg) and its alloys show excellent potential as orthopedic implantable materials, with their in vivo degraded Mg ions (Mg 2+ ) known to promote the growth of new bone. However, the swift corrosion process during implantation has greatly hindered its clinical applications. A method to counter the high rate of corrosion is to coat Mg substrates on surfaces with a thin layer of biodegradable polymer. Although such a coating reduces the long-term corrosion rate, it also prevents the short-term release of bone-simulating Mg 2+ after orthopedic operations. To balance these contradicting short- and long-term characteristics, we present a polymer-inorganic composite coating on pure Mg substrates that enables Mg-based implants to achieve controllable release of Mg 2+ and high corrosion resistance simultaneously. The coatings were fabricated by adding an appropriate amount of inorganic magnesium sulfate heptahydrate (MgSO 4 ·7H 2 O) salt particles into a biodegradable poly(l-lactic acid) (PLLA) polymer matrix, such that they can percolate inside to form an interconnected morphology during the phase separation between Mg salt and PLLA polymers as solvent evaporates during the drying process, resulting in the formation of an organic-inorganic composite coating. The in vitro corrosion tests indicated that the composite coatings with lower Mg salt loading had the best degradation behavior, resulting in controllable release of Mg 2+ and alkaline shift. Cytocompatibility of bare and coated Mg were investigated via MC3T3-E1 preosteoblasts through MTT assay and LIVE/DEAD imaging, along with the observation of cell distribution and adhesion behaviors. The results further demonstrated that the incorporation of a suitable amount of Mg salt particles could further improve the cytocompatibility as compared to the pristine PLLA coating. We believe such fabricated organic-inorganic composite coatings could have great potential for application on Mg substrates to obtain Mg-based biomaterials with higher practical value in clinical treatments.