Chemical forms of Cr, Cu, Mn, Ni, Pb and Zn in the surface sediments of Nansihu Lake and its main inflow rivers are determined by the BCR sequential extraction method. The chemical fractionation composition characteristics, pollution state and potential ecological effects of the heavy metals are discussed. Cr, Cu, Ni and Zn in the sediments of Sihe River, Jinghang Canal, Zhuzhaoxin River, Guangfu River, south area of the Upper Lakes, and Down Lakes mainly exist in residual fraction, which account for 80% - 90% of their content, mainly reflecting the natural characteristics of the heavy metals; but for the sediment of Old Canal, Guangfu River estuary, Baima River, nearly 80% of Cr, Cu, Ni and Zn exist in extractable fractions due to human pollution, especially with high oxidizable and acid extractable fractions content. Pb and Mn mainly exist in residual fraction and oxidizable fraction in the sediment of Nansihu Lake and its main inflow rivers, which account for 95% and 70% of the content of Pb and Mn, similar to other unpolluted sediments. With the mineralization and decomposition of organic matter and change of environment condition of the surface sediment, the extractable fractions of Cr, Cu, Ni, Zn in the polluted sediment can be reactivated, bringing on ecological harm to the water ecological system.