Abstract: Measurements were made on the pH and Eh of the sediments, and Cl-, SO42-, S2-, SiO2, Ca, Mg, Fe, Mn, Zn, Cu, Cd, Ni, Co and Cr of interstitial water of the sediments in Lake Donghu. The pH and Eh of the sediments were related to the degree of water pollution and conditions of aquatic organisms. The average concentrations (ppm) of the chemical components were: Cl-, 25.02; SO42-,28.33;S2-, 1.82; SiO2, 17.12; Ca, 63.84; Mg, 10.39; Fe, 6.68; Mn, 1.20; Zn, 0.31; Cu, 0.15 and Cd, 0.02, respectively. Ni, Co and Cr were undetected in some samples. In the geochemical processes of interstitial water in the sediments of Lake Donghu, the over saturation ratio of CaCO3 to MgCO3 is 44.6, the molar ratio of calium to magnesium in 9 samples was about 3.5. The system of "CaCO3—MgCa(CO3)2—MgCO3" was controled by CaCO3. In the sulphureous system of interstitial water of the sediments in Lake Donghu, SO42- was in the dominant field; the soluble concentrations of some metallic sulfides were greater than the related Ksp. The soluble SiO2 in the interstitial water was mostly in the form of amorphous silica. Hydroxyl on the surface of silicate can absorb metal ions when the silicate is in sedimentation. In this way, metal ions can be accumlated into the sediments for the purification of water.