The microstructure and formation mechanism of hydration products of magnesia/silica fume（MgO/SF） pastes in different Mg/Si molar ratios were investigated by X-ray diffraction（XRD） and ²⁹Si solid state nuclear magnetic resonance（²⁹Si SSNMR） respectively. The results reveals that the formed magnesium silicate hydrates（M-S-H） is a complex structure with a high degree of polymerization，and the formation of M-S-H is related to the formation and decomposition of magnesium hydroxide （MH）. It is inevitable that M-S-H and MH competes for the acquisition of Mg²⁺. MH is mainly formed in the early hydration age of MgO/SF pastes while M-S-H is mainly formed in the late hydration age. The prolongation of hydration time is conductive to polymerization of M-S-H， and the long curing age is good for the conversion of Q¹ to Q² and Q² to Q³. The MgO/SF paste with higher Mg/Si molar ratio promotes the formation of M-S-H， and Mg²⁺ plays the role of dismantling network in long curing age， as the result， the content of chain tetrahedra increases. A large amount of unreacted SF remains in the MgO/SF pastes with lower Mg/Si molar ratio， and Mg²⁺ plays the role of network supplementation in long curing age， which is used to construct layered silicate skeleton of M-S-H. M-S-H is a complex amorphous structure with mainly layered tetrahedra and chain tetrahedra and branching chain tetrahedra.