The microstructure of magnesium silicate hydrates pastes in different Mg/Si molar ratios was investigated by X-ray diffraction, Fourier transform infrared spectroscopy, solid-state nuclear magnetic resonance and thermogravimetric analysis, respectively. The results reveal that the formed magnesium silicate hydrates (M-S-H) is a main structure of layered tetrahedra with some chain tetrahedra and branching chain tetrahedra. The layered tetrahedra is dominated by Q³_a at a higher Mg/Si molar ratio, and the layered tetrahedra is Q³_b at a lower Mg/Si molar ratio. The contents of Q¹, Q² and Q³, especially Q² in the system of MgO-SiO₂-H₂O, all increase as the Mg/Si molar ratio increases. Also, the content of magnesium hydroxide (MH) increases, the content of unreacted Q⁴ silica decreases rapidly, the degree of hydration reaction increases, the degree of gel order and the degree of gel polymerization decrease. At a long age, the decomposition rate of MH increases, and the complexity of the gel structure increases, and the prolonged hydration time is conducive to the formation of diversified layered silicon-oxygen tetrahedral structures and the occurrence of M-S-H polymerization. The water structures of M-S-H are free, confined and structural water. The hydroxyl group is bonded with Mg²⁺ ions or combined with adsorbed water. Chain tetrahedra and branching chain tetrahedra are linked to hydroxyl groups, but Q³ silicate sites are not directly linked to hydroxyl groups. The molar ratio of Mg and Si or structural water and Si in a gel is a fixed value.