The peak area of hydroxyl stretching vibration was quantified to study the diffusion process of water in asphalt by Fourier transform infrared attenuated total reflectance spectroscopy(FTIR ATR), and the influences of temperature, pressure and asphalt aging were also investigated. The results show that there is a correspondence between the area of the OH stretching and the water content diffusing into asphalt. Moreover, the diffusion process can be divided into two stages, in the first stage the priority is given to absorbing free water with a higher diffusion coefficient, and in the second stage water molecules and asphalt molecules or polar group are combined, the diffusion coefficient decreases correspondingly. The increase of temperature and pressure enhances the diffusion ability of water in asphalt. Meanwhile, aging can reduce the resistance of asphalt to water diffusion and accelerate the diffusion of water, especially in the first diffusion stage.