As chloride ion diffusion in concrete theoretical model without considering the shortcomings of the concrete maturity,the improved maturity model is used to establish the relationship between the diffusion coefficient of concrete maturity. With the mathematical derivation, getting new chloride ion diffusion theoretically, which considers the maturity of the concrete, the chloride ion binding capacity, the degradation effects of chlorine ion diffusion equation,and consequently broadens the scope of the fick's second law of diffusion, besides, its form and solution completely consistent with the one in the second law of fick equation. By the calculation example,it can be found that the model is more accurate than the traditional ones, and it also applies to the high content of high-performance concrete.