Based on the field experiment of concretes exposed to marine tidal environment for 60，120，240，360，480，600d, the chloride diffusion coefficients of concretes with 5 water cement ratios were determined. According to the chloride diffusion coefficients of concretes exposed for 60，120，240，360，480d, the time variation model of chloride diffusion coefficient in concrete was deduced under natural tidal environment. Based on the results above, chloride diffusion coefficient and free chloride content of concrete exposed for 600d were calculated by the time variation model, and the adaptability of the time variation model was verified. Results indicate that the higher water cement ratio of concrete is, the lower attenuation coefficient of chloride diffusion coefficient is, the poorer reduction effect of long term chloride diffusivity in concrete is. Thus reduction of water cement ratio of concrete is effective to reduce long term chloride diffusivity in concrete. The decrease range of chloride diffusion coefficient in concrete exposed for 600d is very narrow, and the chloride diffusion coefficient of concrete is stable basically after 600d. The time variation model of chloride diffusion coefficient in concrete can better reflect the chloride erosion process in concrete under natural tidal environment.