In order to verify whether a conventional wastewater treatment process can provide sufficient protective screen of heavy metals, a long-term regular monitoring of the influent, effluent, and water at different stages of a domestic wastewater treatment plant was conducted, targeting As, Cd, Cr, Cu, Hg, Pb and Zn. For most of the heavy metals, more than 50%’s removal efficiency could be obtained by the whole treatment process, in which a fraction of heavy metals was removed by the primary settling tank, while the following biological unit made great contribution to heavy metal removal. Through a series of adsorption experiments using quartz sand, kaolin of known size distribution as inorganic adsorbents and laboratory-cultured activated sludge as organic adsorbent for adsorbing model of As and Cr, it was found that the adsorption of heavy metals by the inorganic adsorbents well followed by the Langmuir isotherm with the saturation capacity proportional to the specific surface area of the solid particles, indicating a single-layer adsorption property. However for the activated sludge, its adsorption isotherm showed a property of multilayer adsorption and the adsorption capacity was one order higher than the inorganic particles. The different behavior between the activated sludge and the organic particles for heavy metals adsorption explained the mechanism of heavy metals decay in the different stages of the conventional wastewater treatment plant and indicated that the activated sludge in the biological treatment plant played an important role for heavy metals removal. Models obtained from the adsorption experiment were practical, and can provide an approach to predicting the contents of heavy metals in the WWTP’s effluent.