Objective:To study the biotoxicity of chlorobenzene and some of the intermediate products. Methods:Mixture toxicity index(MTI) was used to evaluate the joint toxicity. Results:The results showed that the intermediates para benzoquinone had a higher toxicity, while the toxicity of the other intermediate products was much lower than that of chlorobenzene. The toxicity of intermediate products such as p-chlorophenol, hydroquinone and phenol was all decreased, and with the chloride ion substituted by the OH radicals, the phenyl broke down to the small molecules like organic acids, the toxicity of which was even more lower. Conclusion:The joint effects of the intermediate products were mostly additive action and synergistic action.