Acute toxicity tests of the effluents from both inlet and outlet of the treatment system in a chemical plant in Jiangsu Province were conducted with Daphnia magna. The results showed that both effluents were toxic to this species. Toxicity identification evaluation (TIE) was used to identify the key toxicants in the effluent before and after treatment. The results indicated that Cu2+ was the key toxicant causing the toxicity of input water, and some metals and polar organics were the coexistent toxicants in the water. The toxicity of output water was caused by some toxic organics, which were easy to be oxidized under acidic conditions. The results also showed that the toxicity removal efficiency reached 77.6% through the treatment process. It could be seen that the effluent treatment process in this plant had a higher efficiency of removing heavy metals, but the removal rate of organic compounds was quite low. It was concluded that the production techniques and the effluent treatment process of this plant were not perfect, and needed to be improved.