Coal-series metakaolin of 600 mesh (23 μm) and 1 000 mesh (13 μm) was tested according to the concentration of 0%, 5%, 10% and 15% (mass fraction) mix into concrete, and the influences of coal-series metakaolin fineness and content on the mechanical properties and microstructure of concrete were studied by strength, XRD, TG-DTG, SEM-EDS and nitrogen adsorption tests. The results show that the incorporation of metakaolin significantly improves the mechanical properties of concrete. When the fineness of metakaolin is 1 000 mesh and the content is 15%, the compressive strength of concrete is the largest, and the 90 d compressive strength reaches 81 MPa. The hydration products are mainly composed of calcium hydroxide, ettringite, hydrotalcite and calcium silicate hydrate (C-S-H) gel, etc. The addition of metakaolin do not change the type of hydration product, but increases the content of C-S-H gel in the hydration product, while reduces the content of calcium hydroxide. Metakaolin and cement hydration product calcium hydroxide undergo secondary hydration to form C-S-H gel and improve the compactness of concrete, which is the main reason that metakaolin can enhance the mechanical properties of concrete.