To better understand and then to predict the ecological risk of arsenic influenced by phosphorus regimes in freshwater environment, the growth differences of Microcystis aeruginosa (M. aeruginosa) as well as its responses to the toxic stress of arsenate [As(Ⅴ)]were investigated under orthophosphate (DIP) and dissolved organic phosphorous (DOP) [adenosine triphosphate (ATP-P) and β-sodium glycerophosphate (β-P)] culture conditions. The results showed that M. aeruginosa grew and proliferated without any differences under DOP and DIP conditions during the first 5 days, while slower growth rates were observed in DOP conditions. Herein, the cell optical density (D) of M. aeruginosa in β-P and ATP-P conditions was 78.0% and 75.4% of that under DIP condition respectively on the 7th day. The negative correlations between actual quantum yield (Yield) and Chlorophyll a (Chl-a) and D indicated that Yield should not be used as a stable index to reflect the nutrition conditions of algae. However, Yield was a sensitive index to exactly represent the responses of M. aeruginosa to As(Ⅴ) toxicity, which showed significant differences under different phosphorus regimes. Obtained by D, Yield and Chl-a, the 96h EC50 was in the order of DIP > β-P > ATP-P. The similar toxic tolerant abilities to As(Ⅴ) of M. aeruginosa under β-P and ATP-P conditions were lower than that of EC50 under DIP conditions by one to five orders of magnitude.