Concept and the present situations of polders in the Taihu lake region are introduced in this paper. Polders have been built to defense floods effectively, but it has made the drainage of basin not smooth. Built polders have effects on the flood regime of Taihu Lake region, i.e. (1) The areas of water surface and capacities of store water are reduced; (2) Natural water networks are changed; (3) Run-on and runoff are affected by human activities, the increasing of pump power made runoff time much shorter than before. Based on the analysis, some methods of flood control in polders are given.