The invention relates to a water alkalinity on-line detection device, a water alkalinity on-line measuring device based on solution image technology and a measuring method thereof, and is characterized in that according to the invention, based on the detection of solution image RGB values during the titration of water to be detected by a HCl solution, an image RGB value-titration solution volume measuring curve is plotted, and the water alkalinity of the solution to be detected is calculated according to the titration solution volume corresponding to an inflection point of the image RGB value measuring curve. With computer image processing technology, the invention establishes a mathematical model of solution RGB values with the change of the titration solution volume during titration, determines the titration end point automatically, establishes a color constancy algorithm model based on a neural network algorithm, and thus increases the precision and reliability of alkalinity measurement. With the device of the invention, on-line measurement of phenolphthalein alkalinity and total alkalinity can be performed optionally, and the operation is convenient. The device is applicable to industry of electric power, petroleum, chemical industry, metallurgy, etc.