Understanding the mechanism of tricalcium silicate (C3S) hydration is significant for controlling the process of cement hydration. In this work, the adsorption mechanism of water on the C3S surface is revealed from first-principles calculations. We find: (1) the adsorption energy increases with the increase of electron transfer at water/C3S interface and strength of total Ca-Ow bonds; (2) the mechanism of adsorption of water on C3S surfaces is due to the electron transfer from VBM of C3S surface to LUMO of water molecule, raising the Fermi level and shifting the bonding molecular orbital downward; (3) the water becomes more structured and motionless as being closer to the surface; and (4) The stability of the dissociative and molecular adsorption is reverse for isolated and bulk water due to the different reaction pathways. Our findings may shed light on the fundamental understanding for the initial stage of C3S hydration.