By using RS and GIS techniques, the spatiotemporal changes of wetlands in Hangzhou Bay Industrial Belt, one of the most developed zones in Zhejiang Province, from 1990 to 2005 were studied. There was a frequent conversion between the wetlands and other land use types and between the wetlands themselves, mainly manifested in the conversion between wetland and farmland, and from wetland to construction land and from tidal flat to aquiculture area. The comparative advantage of other land use types and the policy of cultivated land's requisition-compensation balance decided the inherent mechanisms of these spatiotemporal changes. Driven by the aquaculture's comparative advantage to traditional agriculture, large areas of inland farmland and of the tidal flat along the coast of Hangzhou Bay were reclaimed into aquiculture area, and the rapid expansion of construction land, limited land resources, and the implement of cultivated land's requisition-compensation balance policy induced the wetlands being occupied.