As the silicon counterpart of graphene, silicene has a great potential in reinforcing cement-based materials due to its outstanding physical properties. In this paper, the surface structure, bonding, mechanical performance and failure mechanism of silicene reinforced calcium-silicate-hydrate composite (C-S-H/silicene) with different humidity were investigated by molecular dynamics simulations. It has been demonstrated that the incorporation of silicene nanosheet can effectively bridge the upper and lower layers of C-S-H and enhance the strength and stability of the interlayer region of C-S-H via various chemical bonds (e.g. Si-Os-Si covalent bonds). This bridging effect provided by silicene nanosheet significantly improves the strength and plasticity of C-S-H. However, the penetration of water molecules may weaken the stability of these chemical bonds, leading to the degradation of tensile strength. Besides, C-S-H/silicene breaks in the internal area rather than interlayers in a dry state during the failure process. This can be explained that Si-Os-Si bonds are strong enough to connect the silicene sheet and C-S-H layers and hinder the propagation of cracks. Hopefully, the reinforce mechanism of silicene on C-S-H could provide new insights on understanding and predicting performance of silicene under specific conditions.