This paper presents an improved double diffusion model (DDM) for the alkali silica reaction. By this model we could simulate the diffusion process of alkali both in concrete (macro-scale) and aggregate (micro-scale). As the alkali source, either internal or outside cases can be analyzed through this model. Concrete with different size groups of aggregate distribution also can be analyzed. Numerical simulation is utilized to analyze the ion concentration both inside the concrete and aggregate at different moment of penetration.