Algal photosynthesis inhibition method is a potential method for rapid detection and early warning of comprehensive toxicity of water, and stable test conditions are critical to the toxicity test results of the algae photosynthetic inhibition method. In this paper, Chlorella pyrenoidosa is used as the test algae, and according to the characteristic that pesticide Dailon (DCMU) has an inhibitory effect on the photosynthetic activity of algae, the inhibition rates of the four photosynthetic fluorescence parameters (α, Fv/Fm, Yield, and rP) are as the analysis objects. Under the stress of different mass concentrations of DCMU, the influences of the three test conditions, including the mass concentration of the tested algae, the mixed volume ratio of the algae liquid to the sample, and the reaction temperature, on the stability of detecting the comprehensive toxicity of water has been studied separately. The results show that when the mass concentration of the tested algae is in the range of 100~600 μg·L ^-1, the change of the algae mass concentration has no significant impact on the detection of comprehensive toxicity of water. However, the mixed volume ratio of the tested algae solution to the water sample has a great impact on the sensitivity of toxicity detection. When the mixed volume ratio of the tested algae solution to the water sample is 2∶8, it is more conducive to improving the sensitivity of aquatic toxicity detection. In addition, the reaction temperature also has an important effect on the detection of comprehensive toxicity of water, and when the temperature is between 20 ℃ and 35 ℃, it is more conducive to improving the detection accuracy of comprehensive toxicity of water.