In recent years, considerable attention has focused on the use of Mg-Zn-Ca as a bioalloy for bone implantation. The microstructures, compositions and phases of the as-cast Mg-4Zn-xCa (x=1, 2, 3, wt.%) alloys were characterized with high precision via differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an energy dispersive spectrometer (EDS). The macroscopic corrosion rates of the three alloys were measured via hydrogen precipitation experiments, and the corrosion order of the phases in the alloys was determined via dynamic corrosion observation techniques. The intrinsic connections among the phase composition, phase corrosion order and corrosion rate of the alloys were further analysed, and the effects of different Ca contents on the phase changes, phase forms and corrosion mechanisms of the alloys were explored via energy spectrum analysis. The results demonstrate that the microscopic corrosion order of the alloy is Mg2 Ca phase > α-Mg matrix > Ca2 Mg6 Zn3 phase. In the presence of the Ca2 Mg6 Zn3 phase at the grain boundaries, corrosion of the α-Mg matrix is effectively blocked by the Ca2 Mg6 Zn3 phase. However, when the Mg2 Ca phase and the Ca2 Mg6 Zn3 phase are distributed alternately at the grain boundaries, the preferential corrosion of the Mg2 Ca phase destroys the reticulation structure of the second phase at the grain boundaries, thereby preventing the corrosion extension of the α-Mg matrix. Consequently, from a macroscopic perspective, the corrosion rate of the alloy containing the Mg2 Ca phase is higher. The microscopic composition of the second phase and its distribution at the grain boundaries are therefore identified as the key factors determining the differences in the macroscopic corrosion rates of the Mg-Zn-Ca alloys.