The present invention discloses a method for rapidly quantitating 
Microcystis aeruginosa in water. The method comprises: preparing a group of 
Microcystis aeruginosa liquids with different concentrations, determining the 
absorbance respectively at wavelengths of 220 nm, 360 nm, 420 nm, 520 nm, 640 nm and 680 nm through a spectrophotometer, and counting the 
algae cells of each sample by using a 
blood cell plate counting method to obtain the number of the 
algae cells per 
unit volume of the 
algae liquid; determining the 
absorbance of a blank sample at wavelengths of 220 nm, 360 nm, 420 nm, 520 nm,640 nm and 680 nm, and drawing a 
calibration curve according to the 
absorbance and the 
cell concentration, or obtaining regression equations corresponding to the wavelengths through 
linear regression; and diluting a 
Microcystis aeruginosa-containing sample to be measured, determining the absorbance of the algae liquid at the specific wavelengths by using the spectrophotometer, and calculating thecell concentration of the 
Microcystis aeruginosa in the sample to be measured according to the established 
calibration curve or regression equation. According to the present invention, with the method, the 
biomass is determined, such that the important significance is provided for the monitoring and early warning of 
algal bloom.