A geometric model of capillary tubes was proposed to characterize the pore structure of concrete. The pore structure parameters, such as pore number, porosity and tortuosity, were represented as functions of the fractal dimension. Depending on the relationship between water saturation and effective space for gas diffusion, the gas diffusion model in concrete was established. Numerical simulations were carried out to investigate the effects of water saturation, water cement ratio and environmental temperature on the gas diffusion coefficient in concrete. Results show that, water saturation is the main parameter influencing the gas diffusion coefficient in concrete. When the water saturation is of 85%(by mass)，the effect is more significant.