Microcracking induced by mechanical damage and moisture conditions both significantly modify the transport paths of aggressive species in cementitious materials. This study proposes a microscopic model to evaluate the coupling effects of microcracks and moisture conditions on gas permeability in cementitious materials accounting for its heterogeneous microstructure. The 3D microstructure of hydrating cement paste was simulated using a voxel-based hydration model, based on which the fracture process of cement paste under the uniaxial tensile loading was simulated using a finite element model. Subsequently, treating the damaged cement paste with various damage degrees as input, a lattice Boltzmann modelling framework was present to mimic the gas permeability of partially saturated cement paste considering the moisture distribution in its pore structure. Results indicate that gas permeability of cement paste with a lower water-to-cement ratio is more sensitive to the damage and the relative gas permeability is increased with the increase of damage degree. With the increasing water saturation level, the permeation paths for gas in cement paste are declined, while the air-filled microcracks as permeation paths are not significantly influenced. At a given damage degree, the increasing water saturation level leads to an increase in the relative gas permeability.