The invention relates to a water harness on-line measurement device. A measurement method of the water hardness on-line measurement device based on solution image technology is characterized by adding dropwise EDTA into a quantitative water solution with the pH of 10.0 plus or minus 0.1 by using a constant flow adding pump with chrome black T or acid chrome blue K as indicator, L-cysteine hydrochloride and triethanolaminesolution as screening agent and ethylenediaminetetraacetic disodiumsalt solution as titrant, recording the volume of added EDTA (Ethylene Diamine Tetraacetic Acid) solution by a computer, simultaneously collecting water solution image colors by using a CCD (Charged Coupled Device) image sensor, displaying image color RGB (Red, Green and Blue)-titrating solution volume real-time curve on a computer screen, and calculating the water hardness according to the titrating solution volume corresponding to the inflection point of the RGB-titrating solution volume curve, wherein the detection precision is improved by adopting a color constancy algorithm based on a neutral network to reduce the interference of factors such as a light source, sensor noise and the like. The invention have the choices of high hardness and low hardness on-line measurement, is simple and convenient in operation, and can be used in the industries of electric power, petroleum, chemical industry, metallurgy and the like.