Alkali metal ions as some impurity ions in cement raw materials can affect the structure and properties of clinker minerals. The effect of alkali metal ions on the hydration activity and mechanical properties of ternesite was investigated with alkali-doped ternesite synthesized as analytical reagents by isothermal calorimetry, comprehensive thermal analysis, scanning electron microscopy and 29Si solid state nuclear magnetic resonance. The results show that the solid solution of alkali metal ions in the crystal structure of ternesite reduces the crystallinity of formed crystals, forms the crystal defects, effectively improves the early hydration activity of ternesite, and promotes the rapid development of its early mechanical properties. Meanwhile, the addition of alkali metal can change the microstructure and structure of calcium silicate hydrates (C-S-H). Among them, Li2O addition stabilizes the flocculent C-S-H, while Na2O and K2O addition can induce the fibrous growth of C-S-H. The degree of polymerization and mean silicon chain length of C-S-H increase due to the addition of alkali metal.