Waterlogging in the middle and lower reaches of the Yangtze River is very severe. In the paper, on the basis of the two-dimensional flow model and by applying the finite volume method and unstructured mesh, the waterlogging and drainage of the Datong Lake-polder with different precipitations and floods were studied. The spatial distribution of waterlogging in the Lake-polder area and the ratio of waterlogging were gained. The negative influence of the floods on the waterlogging was analyzed quantitatively. The model results can provide technological support for the waterlogging forecasting and assessment.