The diffusion system of recycled asphalt was studied by using materials studio （MS） molecular dynamics simulation software. Based on the four component test of asphalt， 12 molecular models were selected to construct the molecular groups of regenerator， the composite water aged asphalt and the composite UV aging asphalt. The diffusion coefficient of the diffusion system was obtained by simulating the aging asphalt diffusion system under different simulation time， simulated temperature and asphalt composition. The results show that the diffusion coefficient of the regenerator in the aging asphalt diffusion system decreases with the increase of the simulation time， and the diffusion coefficient of the composite UV aging asphalt diffusion system is smaller than that of the composite water aging asphalt diffusion system； with the increase of diffusion temperature， the growth rate of diffusion coefficient gradually slows； the diffusion coefficient of aromatic and saturated components of oil component in the regenerant is larger than that of polar component gum and asphaltene. Because the aromatic components are mostly chain and the relative molecular weight is small， the diffusion rate of the oil component is the fastest among the four components.