A novel grain refinement method using a CaMgSn intermetallic phase was investigated via adding a Mg–CaMgSn master alloy to an AZ31 magnesium alloy. The results showed a remarkable grain refinement effect in as-cast AZ31, with the average grain size reduced from approximately 260–93 μm after adding 0.45 wt.% CaMgSn particles. The added CaMgSn phase was found in grain interiors and acted as heterogeneous nucleation sites during solidification. The edge-to-edge matching model confirmed a low mismatch value of 2.3% for the ${\left\{0002\right\}}_{\mathrm{Mg}}/{\left\{211\right\}}_{\mathrm{CaMgSn}}$ close-packed plane, suggesting that {211}_CaMgSn was the possible matching plane for Mg nucleation. The microhardness and compressive yield strength were also improved by adding CaMgSn particles, confirming that adding Mg–CaMgSn master alloy was an effective method for refining the microstructure and improving the mechanical properties of Mg–Al-based alloys.