Abstract: Cement-based materials in deep sea area are subjected to the double effects of water pressure and chloride ion concentration gradient, which may result in different transmission law of chloride ions in deep sea area with in the general environment. Taking the cement mortar test block as the research object, the transmission mechanism of chloride ion in cement-based materials under the coupling effects of hydrostatic pressure field and chloride concentration field was verified by the simulation technology based on COMSOL software and the corresponding physical experiments. The results showed that the penetration depth of chloride ion is greatly affected by the change of water-cement ratio under hydrostatic pressure, and it increases rapidly with the increase of water-cement ratio of cement-based materials. At the hydrostatic pressure of 0.5 MPa, the penetration depth of chloride ion increased by 28.1% and 52.9% respectively when the water-cement ratio of mortar test block increased from 0.35 to 0.45 and 0.55. The penetration depth of chloride ion in mortar test block with the same water-cement ratio changes obviously under different hydrostatic pressure. When the water-cement ratio of mortar test block was 0.35, with the hydrostatic pressure increasing from 0 to 0.5 MPa and 1.0 MPa, the ultimate penetration depth of chloride ion increased by 43.1% and 123.6% respectively at the age of 30 days.