Abstract: The ecological risk of a Microcystis bloom was evaluated on the basis of the ecotoxicological risk assessment method. The effects of Microcystis bloom on benthic fauna, zooplankton, fish, and submerged macrophytes were assessed by using acute and chronic exposures experiments and the findings reported by previous studies. The algal cell densities were calculated by using toxicity percentile rank (TPR) method, and aquatic ecological risk assessment for Microcystis bloom was performed by using risk quotient approach. The criteria required for acute and chronic ecological risk assessment of Microcystis bloom were defined based on the algal cell density, algal bloom coverage in lake, as well as duration of algae blooms. The acute ecological risk was classified as low-, moderate-, and high-levels. A Microcystis bloom with cell density less than 3.4×10⁶ cells/L was defined as low risk-level, with cell density at the range of 3.4×10⁶—3.4×10⁷ cells/L was denoted as moderate-risk level, and with cell density higher than 3.4×10⁷ cells/L was defined as high-risk level. Similarly, chronic ecological risk of Microcystis bloom was also classified as low-, moderate-, and high-levels of risk. The Microcystis bloom with cell density less than 1.1×10⁶ cells/L was classified as low risk, cell density between 1.1×10⁶—1.1×10⁷ cells/L was classified as moderate risk, and the cell density greater than 1.1×10⁷ cells/L was classified as high risk. In practical applications, cell density, algal bloom coverage, and duration of algal blooms are essential factors to formulate a robust ecological risk assessment strategy for Microcystis bloom in Erhai Lake. In presence of moderate-risk level, early warning of Microcystis bloom should be alarmed, and improvement of water quality or implementation of biological control should be carried out. Under the high risk level, emergent removal or control of algal bloom should be undertaken as well.