Magnesium (Mg)-based biometals are increasingly becoming a promising candidate of the next-generation implantable materials due to their unique properties, such as high biocompatibility, favorable mechanical strength, and good biodegradability in physiological conditions. However, the swift corrosion of Mg, resulting in early loss of structural support, has posed an enormous challenge in clinical application of Mg-based implants. To overcome these limitations, herein we developed a novel method, which combines the traditional dip-coating with nonsolvent induced phase separation (NIPS), to fabricate biodegradable PLLA coatings with controlled membrane morphology on pure Mg rods. Unlike the conventional dip-coating, where the polymer solution on the Mg substrates is left to evaporate directly under proper atmosphere, in NIPS, the polymer solution on the substrates is not left to dry but immersed in a nonsolvent of the PLLA, leading to the precipitation of polymer networks. Our results demonstrated that various polymer coatings with different morphologies and inner structures could be easily fabricated by a careful selection of nonsolvents. In comparison to dense PLLA coatings obtained from conventional solvent evaporation, PLLA coatings with a dense surface and porous inner structure were obtained when hexane and petroleum ether were used as the nonsolvents, while PLLA coatings with a completely porous structure were obtained when polar acetone and ethanol were chosen. The electrochemical corrosion tests and immersion tests further showed that all polymer coatings could significantly improve the corrosion resistance and suppress the corrosion rates of the substrates. However, PLLA films obtained via NIPS had much lower pH changes and slower Mg2+ release, implying better protective effects of the fabricated coatings. Based on results of all experiments, a new process for the corrosion mechanism of Mg implants during immersion has also been proposed in this work.