The exchange of water mass into and out of Dongsha Atoll is analyzed based on field measurements, in the region of 116.7-116.92E 20.58-20.78N during year 2008-2010. Measured parameters include sea level, currents, waves, water temperature and bottom depth. The results show that (1) tidal current is the major driven force of water exchange in the Dongsha Atoll. (2) The major water pathway is the two channels at the northwest and southwest by the side of the Dongsha island. The north channel is more important than the south. (3) The volumn transport of wave contribution is small. (4) The estimated residual time of water in the atoll is about 2 days.