Abstract: Taking CaCO3 as a refining agent, the influence of CaCO3 on the microstructure of AZ31 magnesium alloy was investigated. The results showed that the best refining effect is obtained if adding 0.5wt% CaCO3 to the alloy at 760°C held for 10 min, i.e., the average grain size of α-Mg decreases to 209 μm from about 570 μm of the alloy or 63.3% is reduced. Based on the analysis of EDS, the calculations of binding energy and Gibbs free energy proved that, the grain refining mechanism is resulting from a part of Al4C3 particles as heterogeneous cores generated after the reaction of CaCO3, whereas the rest of Al4C3 particles are pinned to the grain boundary to retard the grain growth, and Al element is then quickly pushed to the grain boundary along the solid-liquid interface to generate the β-Mg17Al12 phase growing along the grain boundary, thus immobilizing the grain boundaries with the distributions of alloying elements changed.