The liquid level measurement of the connecting pipe is an important way to detect the deflection of the bridge. Abstract: Aiming at the problems of insufficient real-time performance and measurement accuracy of traditional liquid level measurement methods for connected tubes, a liquid level measurement system for connected tubes was studied based on machine vision, which was composed of liquid level observation tube, light source, parallel light plate, industrial camera, microprocessor and router. The edge contour of the liquid level image was obtained by image preprocessing, and the sub-pixel edge detection algorithm was used to accurately locate the bottom coordinates of the edge contour of the liquid level image. A method for calibrating and measuring the liquid level height based on the principle of relative motion was proposed. The function relationship between the bottom coordinates of the edge contour of the liquid level image and the liquid level height was obtained by fitting with the least square method. Finally, the feasibility, accuracy and dynamic response of the liquid level measurement method are verified by static measurement test and pseudo-bridge vibration test in the laboratory. The maximum absolute measurement error is less than 0.5mm, which can meet the engineering application needs of deflection measurement of connecting pipe bridges.