The isothermal exposure tests for 270 d at different air pressure (i.e., 101 kPa, 60 kPa, and 20 kPa) and relative humidities (i.e., 43%, and 98%) were carried out for hardened cement pastes. Effect of air pressure on the moisture content and the chemically bound water content in hardened cement pastes was investigated. The moisture transfer characteristics at different low air pressures were analyzed by a proposed numerical model. The results show that a low air pressure promotes the water loss within the hardened cement pastes during the isothermal drying process, resulting in a slight decrease in the chemically bound water content. Under the isothermal wetting condition, the mass of hardened cement paste grows at a logarithmic rule due to the moisture diffusion into the pore network, the decrease of air pressure significantly improve the early-stage mass growth rate and the final mass gain. The numerical approach on the moisture transfer indicates that the increases of water vapor diffusion coefficient, mass transfer coefficient, and intrinsic permeability of hardened cement paste are the main causes of moisture transfer behavior during the isothermal drying and wetting processes.