The invention relates to a colorimetric detection method of sugar content of water, which comprises the steps of: under the condition of concentrated sulfuric acid, taking alpha-naphthol as a color-developing agent to prepare furfuraldehyde and derivatives thereof under the action of sugar and the concentrated sulfuric acid, and further performing condensation of the furfuraldehyde and the derivatives thereof and the alpha-naphthol to obtain purple complex compounds; and measuring reactant and recording change of intensity of transmitted light of aubergine condensation compounds absorbing absorb light with wavelength of 595 nanometers by a photoelectric colorimeter to obtain a response curve with corresponding peak height and peak width, and comparing the peak height of the curve with that of a light absorption curve of master standard water with a known sugar concentration to obtain sugar content values in water samples through calculation. The method has the advantages that: the on-line detection of the content of sugar with low content in water or solution can be performed; the process is simple, convenient and short; the detection data is accurate and reliable; the sample injected for one time of detection is only 50mu m to 1ml; only a very small amount of reagent is consumed each time; the operation cost is low; and the reagent is safe and nontoxic.