A survey has recently been done in Jiaxing City of Zhejiang Province at the Yangtze River Delta to determine changes in soil biochemical properties in paddy soils with the traditional cropping system of rice-wheat rotation altered into vegetable cultivation, including continuous plastic mulching vegetable cultivation, and plastic greenhouse vegetable cultivation. Results show alteration of the cropping system induced soil acidification and soil pH dropped to an extent varying with type of the soil (Percolating paddy soil, Permeable paddy soil, and Waterlogged paddy soil). The activities of several soil enzymes including dehydrogenase, urase, inverase and phosphase in soils under plastic mulching vegetable cultivation (MVC) were all lower than under rice-wheat rotation (RWR), and under plastic greenhouse vegetable cultivation than under MVC and RWR. The decreases in biochemical properties in the soils under continuous vegetable cultivation, especially under plastic greenhouse vegetable cultivation, might be caused by soil acidification and overuse of nitrogen and phosphorus fertilizers.