A field investigation was carried out in Beibu gulf and the free chloride concentration in the concrete samples was determined. The results showed that at given depths, the chloride concentration reached the highest in the splash zones and was the second in the tidal zones and lowest in the atmospheric zones. Similarly characters were observed by the fitted surface chloride concentration and the chloride diffusion coefficient. It can be concluded that splash zones affect concrete structures more harshly than tidal zones and atmospheric zones. Chloride diffusion coefficient decreases with time passing by, and age factor obeys the normal distribution. In atmospheric, tidal and splash zones, the mean values of the age factor are 0.19, 0.36 and 0.43, respectively. The investigation shows that the timedependent property of chloride diffusion coefficient has to take into account in durability assessment.