Based on a self-developed high-permeability water pressure loading test system, the moisture content change law of three different strength grades of concrete under four different high-permeability water pressure environments after continuous exposure for 3, 10, 24, 72, and 240 hours was studied. The pore structure characteristics of concrete under high-permeability water pressure environments were measured using the mercury intrusion porosimetry, revealing the microscopic mechanism of water content evolution in concrete under different water pressures. A prediction equation for concrete moisture content under different water pressure was constructed. The results show that the concrete moisture content increases rapidly with the extension of the exposure time under the high-permeability water pressure. The higher the permeable water pressure and the lower the concrete strength grade, the shorter the time required for the concrete to reach saturation and the higher the saturation moisture content. The saturation moisture contents of C15 concrete under 1, 2, and 3 MPa permeable water pressures are 1.12, 1.13 and 1.19 times that under 0 MPa, respectively.