Abstract: The tide and currents in Daya Bay was simulated by a 3D numerical estuarine and coastal ocean model. Lagrangian particle tracking and matter transportation and diffusion were applied in water exchange study. Daya Bay was divided into seven parts to calculate the averaged water residence time and refresh time in the whole bay as well as in each part. The structure of the tidal residual currents, as well as landform and depth play decisive roles in water exchange in Daya Bay. The result showed that the discharge of particles was faster in south of Daya Bay and Dapengao and slower in Yalin gulf, north of Daya Bay and Fanhe gulf as indicated by lagrangian particle tracking. Water refresh time was shorter in southeast of Daya Bay than in middle east, southwest of Daya Bay, Fanhegulf, north of Daya Bay, Dapeng'ao and Yalin gulf.