Chlorella pyrenoidosa is used as the subject and the photosynthetic activity parameters of this type of algaes are used as a toxicity evaluation index. The responses of different photosynthetic activity parameters to the acute toxicity of phenol are studied over 72 h and 70 min. The results show that phenol has significant inhibitory effects on the photosynthetic activity parameters Fv/Fm, rP, α, Ek, and JVP II. Further, under the acute toxicity of phenol, the toxic responses of the aforementioned photosynthetic activity parameters are stable over a short time, where 5 min can be used as a response time for the toxicity analysis of phenol. Finally, the photosynthetic parameters Fv/Fm, rP, Ek, and JVP II show a good dose-response relationship to phenol toxicity and each parameter responds differently to the toxicity of phenol. Across a concentration range of 0.2-1.2 g/L, the inhibition rates of Fv/Fm, rP, Ek, and JVP II are -3%-19%, 9%-67%, 15%-74%, and 17%-37%, respectively.