Based on cluster analysis，the polders in west Taihu Lake are-classified into eight types. Such key elements to cause flood-waterlogging as rainfall，water surface , elevation，dischange capacity, dyke，topography are analysized. An index M and three-day design rainfall are used to study the flood-waterlogging risk of every type of the polders. The main conclusion is 1.Neither all the high lands are softy and nor all the low lands are risky. 2. Human activities have great impacts on flood waterlogging.