Understanding the formation mechanism of calcium silicate hydrate (C-S-H) is pivotal to control the kinetics of calcium silicates hydration and the evolution of C-S-H microstructure. However, the reactions between calcium- and silicate-based aqueous species before or during the process of C-S-H nucleation at molecular level are still unclear. Here, based on ab initio metadynamics simulations, we find that the dimeric interaction between calcium silicate aqua monomers leads to the formation of a calcium silicate aqua Q₀-dimer through an ionic bond between calcium and silicate ions and a reduction of hydration number of the Ca ion, which is prior to the formation of silicate dimers (Q₁). Besides, the first coordination shell of the Ca ion in the most stable calcium silicate aqua cluster has the characteristics of both tobermorite and jennite structures. We further propose a potential formation pathway for C-S-H, where the replacement of water ligands of the Ca ion by silicates initiates the clustering and dimerization of silicate monomers and promotes the formation of C-S-H.