In environmental pollution monitoring at present, methods considering only the single index rather than the combined action of pollutants are mostly used which cannot reflect the comprehensive toxicity of pollutants. Taking chlorophyll-a content and optical energy conversion efficiency as indicators, we analyzed the toxicity of combined mercury (Hg) and DiButyl phthalate (DBP) on Scenedesmus quadricauda. Results showed that the chlorophyll fluorescence parameters of Scenedesmus quadricauda were inhibited either by Hg samples, DBP samples, or by joint samples of Hg and DBP. The chlorophyll-a content and optical energy conversion efficiency decreased with the increase of toxicant concentration, showing an obvious dose-response relationship. A conspicuous synergistic effect of Hg-DBP was found on Scenedesmus quadricauda. The results could provide a theoretical basis for monitoring the comprehensive toxicity of pollutants in water. 