Abstract: There were many intermediate products in the process of nitrobenzene wastewater pretreatment. Some of the intermediate products were more toxic than nitrobenzene, which had bad effect on the microbe in biological treatment. So the toxicity assessment was necessary. The single toxicity and joint toxicity of nitrobenzene and its primary intermediate products (onitrophenol, mnitrophenol and pnitrophenol) in wastewater were studied by using the method of photobacterum phosphoreum in this paper. The joint toxicity of multiple mixtures was evaluated by three evaluation methodology: Toxic Unit (TU), Additive Index(AI) and Mixture Toxicity Index(MTI). The results suggest that the -log EC50 value of the single toxicity for nitrobenzene, onitrophenol, mnitrophenol and pnitrophenol is 3.21, 3.40, 3.30 and 4.15, respectively. The joint toxicity of multiple mixtures is primary additive action or synergistic action.