The invention discloses a grinding fluid used in an on-line electrolyzed metal bond superabrasive grinding wheel and a preparation method thereof, belonging to the technical field of mechanical precision processing. The stock solution of this grinding fluid is composed of 0.1-0.3% sodium molybdate, 2-4% disodium hydrogen phosphate, 0.1-0.5% glycerol, 0.8-1.2% triethanolamine, 0.8-2% borax, 0.12 ~1% sodium sulfite and 0.28~1% red oil are prepared, and the invention is obtained according to the mass ratio of grinding fluid stock solution to water of 1:5~1:20. The preparation method is characterized in that it includes the following steps: ① heating water to 60°C-80°C; ② adding disodium hydrogen phosphate, borax, sodium sulfite, and sodium molybdate into the above water and stirring until dissolved to obtain a solution; ③ adding glycerol, Pour triethanolamine into the above solution and stir until it is completely dissolved; ④Pour in the red oil and dissolve it completely, and cool down to room temperature to obtain the original solution of grinding fluid; invention. It solves the problems of difficult sharpening of resin-bonded abrasive tools commonly used in precision mirror grinding, poor geometric shape retention of the working surface, and short service life.