Objective To solve the problems of traditional chemical conversion coatings on magnesium alloy such as large microcracks and poor corrosion resistance, a novel phosphate /silicon nitride compound coating was prepared in this article. Methods First traditional magnesium phosphate conversion treatment was conducted, and then plasma enhanced chemical vapor deposition technology was used to deposit silicon nitride film layer. The morphology, element distribution, surface potential and polarization curve of the composite coating were analyzed, and compared with those of the phosphate conversion coating. Results Silicon nitride film layer could be selectively priorly deposited on the cracking positions of the phosphate conversion coating, and filled the cracks on the conversion coating layer, forming dense composite coating structure. The surface potential and corrosion potential of magnesium alloy with composite coating structure were significantly higher than those of magnesium alloy treated with traditional phosphate conversion process. Conclusion After preparation of phosphate /silicon nitride double composite coating on the surface of magnesium alloy, the corrosion resistance was significantly higher than that of the magnesium alloy treated with traditional phosphate conversion.