Metakaolin has been generally used as a filler in Portland cement-based materials, and it was proved to be able to enhance the performance of magnesium phosphate cement composites. In this research, metakaolin was introduced into MPC, and compressive strength, tensile bonding strength, porosity and durability of MPC were measured. The microstructure of MPC before and after water immersion was evaluated. The experimental results indicate that the presence of metakaolin can lead to higher plastic viscosity of fresh MPC mortar, 28-day compressive strength, higher tensile bonding strength, better freeze-thaw resistance, better water resistance, lower drying shrinkage and better optimized microstructure. Physical effects of metakaolin including filling effect and inducing effect play the leading role in MPC. Metakaolin particles can fill in the pores or cracks, resulting in the decrease in porosity. In addition, metakaolin particles can provide lots of sites for crystallization of hydration products, inducing in the generation of more homogeneous microstructure. Finally, the dissolution and chemical reaction of metakaolin particles were confirmed by BSE analysis.