The workability mechanism of fresh concrete at the molecular level remains essentially unexplored. To understand the molecular origin for cement fluidity, molecular dynamics and Density Function Theory (DFT) were utilized to construct a shear model of Calcium-Silicate-Hydrate (C-S-H) layers. The structure, dynamics, and reactivity of ultra-confined pore solution between C-S-H gels are systematically investigated. Under shear loading, periodic oscillation of friction force is observed as the typically Couette flow and the interfacial friction force is reduced from 35.2 Kcal/mol·Å to 3.3 Kcal/mol·Å with water content increasing. All of the systems contain breakage of noncovalent bonds of water-Ca and water-water in the lubrication process. But, the obvious breakage of covalent bonds is found in the low-water content system, in which a part of Ca atoms is separated from the C-S-H matrix for lubricating that does not occur in the high-water content system. Furthermore, DFT studies monitor the reaction pathway for the dissociation of water molecules and calcium ions from silicon oxide tetrahedrons. In the energetic respect, it distinguishes covalent-ionic bond transformation and H-bond breakage. The atomic-level mechanisms provide new insights on workability design for fresh concrete.