In recent years, coastal erosion tends to be intensified continuously and has caused serious damage on the economic and social development in the coastal regions. Previous studies on the coastal erosion process and dynamic evolution of the Sanya Bay stayed mostly in a qualitative or semi-quantitative stage. In the present paper, the coastal dynamic evolution process of the Sanya Bay under the actions of waves and tide currents is studied quantitatively based on the theories of coastal dynamic geomorphology and sediment transportation and by means of numerical simulation. The propagation forms and dynamic characters of the waves in the bay are reproduced by numerical calculations. During the high tide the extreme of wave height is relatively larger, with which the water level rises and the fluctuation of the sea surface is obvious and complex in forms, whereas during the ebb tide the extreme of wave height decreases relatively, with which the water level goes down, the sea becomes relatively calm and the wave form is simplified in the offshore area and relatively diversified only in the coastal waters. The simulated results also display well the deposition/erosion process in the area near the coastline of the Sanya Bay. At the western coastal segment of the bay the deposition/erosion phenomenon is obvious due to strong ocean dynamic condition and toward the eastern segment of the coast the deposition/erosion ability decreases gradually. Moreover, the physical process of coastal erosion and evolution of the Sanya Bay is quantitatively simulated based on ocean dynamic mechanism, which would provide a scientific base for the coastal development and utilization and the coastal environment protection in the area studied.