Objective Magnesium alloy possesses a good potential application in the biomedical filed, to overcome its overly rapid degradation speed in human environment, a protective coating with good biological compatibility should be deposited on the surface of magnesium alloy. Methods Ca-P bioactive coatings were prepared on the surface of AZ91D magnesium alloy by electrodeposition method. The deposition conditions were: 0.1 mol/L of Ca (NO3)2, 0.06 mol/L of NH4H2PO4, pH=4.5, deposition voltage 2 V, and deposition time 1 h, 2 h, 3 h, 4 h, respectively. The phase structure, microstructure and chemical composition of Ca-P coating were analyzed by XRD and SEM/EDS, and the polarization curves of Ca-P coating in Hank’s solution were tested. Results The results showed that DCPD coating was prepared on the surface of magnesium substrate by electrodepositon. The microstructure changed obviously with time and when the deposition time was 3 h, the particle size of the coating surface was the most uniform and fine, and the Ca-P ratio was 1.324. Polarization curve results showed that when the deposition time was 1 h the coating had produced protection for magnesium substrate. The corrosion voltage showed an increasing trend with prolongation of time, but the corrosion current reached the lowest. The corrosion potential of the coating was increased by 180 mV, and the corrosion current density was decreased by 3 orders of magnitude compared with the magnesium matrix. Conclusion The corrosion resistance of Ca-P coating was the best when the deposition time was 3 h in this paper.