Experiment was designed to simulate diffusion of regenerant in aged bitumen. Based on the results of softening point test and through regressive analysis, the diffusion coefficient p was used to show the characteristics of diffusion of the regenerant in aged bitumen. The influences of regenerant types, aged degree and environment temperature on the diffusion coefficient p were analyzed. Based on the splitting strength test, the influence of regenerant diffusion on the performance of regenerating bituminous mixture was analyzed. According to the results, the greater the viscosity of regenerant is, the higher the aged degree of bitumen will be, and the lower the regenerant diffusion degree in aged bitumen will be. Meanwhile, increase of environment temperature and diffusion time will enhance the diffusion of regenerant remarkably, and the complete diffusion of regenerant in aged bitumen will be favorable for the improvement of regenerating bituminous mixture strength.