Ta-pong Bay is a shallow coastal lagoon located at the southwestern Taiwan, with only one inlet permanently connecting to the sea. The water exchange, flowing out or into the Bay, is chiefly driven by tidal force. Many kinds of nutrition are gradually accumulated in the Bay, leading to the situation of eutrophication to be more serious. This research utilizes the POM numerical model (Princeton Ocean Model) to simulate the circulation of Ta-pong Bay and long-term water mass transportation. Further, we had used a towed ADCP to survey the flow fields of the Bay and set up a fixed ADCP to measured local velocity for 15 days, for helping us more to understand the flow field. This study using high-resolution 3-D numerical model had been developed with 50m by 50m horizontal resolutions to calculate the problem of water exchange of the lagoon. The model is driven by tidal force, northerly and southerly wind stress. The model results show that the tide-generated force is quite revealed at the tidal inlet where the biggest velocity can researches to 1 m/s during ebb tide, 0.5 m/s during flood tide. The low-frequency motion of the bay is mainly driven by wind stress. The residual current is about 0.03 m/s during blowing northerly wind and about 0.05 m/s during blowing southerly wind. Generally speaking, the average residence time in the south of the lagoon is over 30 days and in the central bay is about 7~12 days and in the north of the lagoon near the tidal mouth is about 1~2 days.