In order to study the asphalt-water mixing process of foamed asphalt，the molecular structure of four components of asphalt was established by using the method of molecular simulation. The asphalt-water separation model was established by using the foamed asphalt with different water content. The results show that，with the addition of water molecules，the diffusion coefficient of asphalt components increases during asphalt -water mixing. When the moisture content reaches 3%，the diffusion coefficient of asphalt components increases obviously. In foamed asphalt，there is repulsion between asphalt and water molecules，which is intensified with the increase of water molecular. With water content of 3% and 4%，the water molecules are evenly distributed in the crystal cell and foamed asphalt foams more evenly.