Abstract: When conductivity of wastewater was higher than 5 000μS/cm, water hyacinths (Eichhornia crassipes Solms.) would die in a few days due to poisoning. When conductivity was over 2300μS/cm, a reduction of growth rate, chlorophyll content and chl. a/b ratio of the plant occurred but the permeability of cell membrame increased. The optimum growth conditions of the plant occurred at conductivity below 1700μS/cm. The toxic components other than salts in Yanshan petrochemical wastewater enhanced the toxicity of salinity. Wheh wastewater was filtered through activated carbon or allowed to degrade naturally, the toxicity would have a remarkable decrease. It should also be pointed out that the purification efficiency of water hyacinths was related negatively with salinity. The purification efficiencies to COD, oil, ammonium-nitrogen, phenol and other pollutants in the wastewaters are different in accordance with the different methods of treatment. Among the experiments, gr/ups- of moderate salinity always revealed maximum purification efficiencies. Under the experimental, condition the more vigorously the water hyacinths grew, the greater reduction of pH in the experimental liquid occurred.