A dicalcium phosphate dihydrate (CaHPO4·2H2O, DCPD) coating is prepared to reduce the biodegradation rate of Mg–Ca–Zn alloy. The substrate is immersed into a solution with Ca(NO3)2·4 H2O 0.1 mol/L and Na3PO4 0.1 mol/L to obtain calcium phosphate coating. Surface morphology is observed by scanning electron microscopy (SEM). Chemical composition is determined by X-ray diffraction (XRD) and EDX. The biodegradable behavior is investigated by immersion tests. The results show that calcium phosphate coating consists of many flake particles and with immersion time increasing, the coating thickness increased and became more uniform and smooth. The coating can reduce the biodegradation rate of Mg alloys in Hank’s.