Understanding the mechanism of tricalcium silicate (CS) hydration is significant for controlling the process of cement hydration. In this work, the adsorption mechanism of water on the CS surface is revealed from first-principles calculations. We find: (1) the adsorption energy increases with the increase of electron transfer at water/CS interface and strength of total Ca-O bonds; (2) the mechanism of adsorption of water on CS surfaces is due to the electron transfer from VBM of CS 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 CS hydration.