The toxicity of heavy metal Hg2+ on alga Microcysis aeruginosa was studied by the change in fluorescence intensity of Microcysis aeruginosa at 435 nm/680 nm which treated with different Hg²⁺ concentrations for 25 min. When the Hg²⁺ concentration is 0.000 5 mg/L, the relative fluorescence intensity of Microcystis aeruginosa (the sample fluorescence intensity minus the blank fluorescence intensity is equal to the relative fluorescence intensity) is negative; while when the Hg²⁺ concentration is 0.001~0.500 mg/L, the relative fluorescence intensity of Microcystis aeruginosa is positive. And the relative fluorescence intensity increases with the concentration of Hg²⁺(0.001~0.400 mg/L), even the concentration of Hg²⁺ and algal photosynthetic signal have a significant positive correlation(r=0.983 3).