Asphalt aging occurs both in the construction period and in the service period of asphalt pavement. It is believed that aging could have a big influence on the rheological mechanical and adhesion characteristics of the asphalt. Asphalt aging is a process associated with a lot of factors as heat, rays, oxygen, and ozone. In the aging process, some chemical compositions of asphalt are subject to be activated by these factors and undergo some chemical changes. A theoretical analysis at a chemical reaction level to the asphalt aging mechanism is presented in this paper. Asphalt aging can be sorted as thermal oxidation aging, ray oxidation aging, and ozone oxidation aging. The thermal oxidation aging usually occurs in the components with chains and double bonds in their molecules. The ray oxidation aging can be found in components with one carbonyl or more carbonyls. Those components with a π key in their molecular structures are prone to the ozone oxidation aging.