As the most important binding phase of cement paste, calcium silicate hydrate can determine the mechanical properties of cementitious materials. Structural, dynamic and mechanical properties of calcium silicate hydrate gel at the colloid level (∼15 nm) were investigated for the first time in light of molecular dynamics. Calcium silicate hydrate models were constructed by aggregation of colloids consisting of layered molecular structures. Five calcium silicate hydrate samples were first constructed by 108 spherical calcium silicate hydrate particles coated by water molecules of thickness from 0 nm to 0.2 nm. The built models including 200000, 300000 atoms were subsequently subjected to uniaxial tension testing to determine the mechanical properties. Structurally, water molecules coating the colloids disjoin the interaction between neighbouring calcium silicate hydrate particles by replacing the stable ionic-covalent bonds with low-strength H bonds. Water molecules in the interparticle region diffuse faster than structural water molecules inside calcium silicate hydrate gel, which reduces the stability of colloid aggregation. With increasing water-coated molecule layers, both the stiffness and cohesive force of calcium silicate hydrate gels are significantly weakened. During the tensile process, the region rich with water-coated molecules become the crack propagation channel, which accelerates the tensile failure of calcium silicate hydrate gel in the saturated state.