Linearity is a very important parameter for seamless steel pipes. A real-time and on-line visual measurement system for seamless steel pipe linearity is presented. The system consists of several structured-light visual sensors. Each sensor can achieve the coordinate of the center of partial steel pipe in its local coordinate system. Through global calibration, all coordinates measured can be transformed into an integrated coordinate system. The linearity error of steel pipe can be assessed. This method can fulfill 100% on-line and real-time linearity measurement. A pair structure-light sensor is designed to improve accuracy and a suspension-wires method for sensor calibration and global calibration is used. Through experiments, it shows that the method not only meets the need of precise calibration but also significantly improves the efficiency and feasibility.