Considering the problem of water exchange in the encircled long basin with one entrance，taking Taiping Bay harbor in Dalian as an example，the layout of opening up a water exchange channel between the bottom of the basin and the sea was put forward. Based on a two-dimensional hydrodynamic and water exchange model，the effects of different opening widths and bottom elevations on water exchange were studied. The results show that the water exchangeability of the basin could be improved by more than 30% after the channel opening at the bottom of the basin，but channel bottom elevations have little influence. The research results are of reference significance for the planning of the encircled long basin.