Existing analytical chloride ion diffusion model can not be used to consider the time invitation effect of temperature and relative humidity in computation period. Based on the Ficks second law, a new method to calculate chloride ion diffusivity was established, which considers not only the influences of water cement ratio, time and chloride ion binding capacity of concrete, but also considers the time variant effect of temperature and humidity. The validity of the new model was verified through the comparison of results between the proposed method and the existing analytic/numerical model. The application scope of this method is wider than that of the analytical diffusion model.