Abstract: A green algae species (Selenastrum capricornutum, FACHB-271) was applied to study the acute toxicity of wastewater from a pharmaceutical manufacturer. Wastewater samples were collected from four treatment process nodes and the effluent of a pharmaceutical wastewater treatment. The acute toxic effects were evaluated by the level of EC50 (concentration for 50% of maximal effect), TU_a (acute toxic unit) and LID (lowest ineffective dilution). The samples from the underground and ground regulating pool were extremely/moderately toxic. Underground (process node 1):EC₅₀=15.12%±3.82%, TU_a=6.6, LID=16; Ground (process node 2):EC₅₀=15.81%±1.04%, TU_a=6.3, LID=16); the primary setting pool wastewater (process node 3), EC₅₀=62.12%±3.83%, TU_a=1.6, LID=8, was moderately/low toxic; the secondary setting pool wastewater (process node 4), EC₅₀=89.10%±1.43%, TU_a=1.1, LID=4, was low toxic; the effluent sample, EC₅₀ > 100%, TU_a＜1.0, LID=1, was not/slightly toxic. The results showed that the toxicity of wastewater samples was reduced gradually after each treatment, and a linear relationship was found between toxicity index and ln ρ(COD_Cr). In addition, the index, LID, can represent the toxic levels of wastewater samples more clearly.