[en] A biodegradable Ca-deficient hydroxyapatite (Ca-def HA) coating has been directly prepared on Mg–Zn–Ca alloy by pulse electrodeposition to improve its corrosion resistance and biocompatibility. However, the formation mechanism of such a Ca-def HA coating on magnesium substrate is still not clear. In this study, the microstructure evolution of the coating was characterized using x-ray diffractometer, x-ray photoelectron spectroscopy and scanning electron microscopy. Thermodynamic and kinetic studies of the precipitation of hydroxyapatite (HA), octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) in the used electrolyte were also carried out. Theoretical analyses illustrate that the precipitation of HA, OCP and DCPD are all possible when the electrolyte pH is higher than 6 at 80 °C, and that the higher the pH value, the more favorable is the formation of HA. Nevertheless, there is mainly poor crystalline Ca-def HA on the substrate when pulse electrodeposition lasts for 5 min, and its crystallinity increases with duration time The direct formation of the Ca-def HA coating on Mg–Zn–Ca alloy is closely dependent on the phase composition and microstructure of the substrate, the deposition parameters and Mg²⁺ ions substitution in HA structure.