In this paper, on-line and real-time laser visual alignment measurement technique is studied to measure the spatial straightness of large objects. The basic principle of the visual alignment measurement is detailed. The mathematics models of visual sensor, measurement system and their calibrations are presented. Based on the technique, an experiment system is developed and the straightness of a 1500 mm long seamless steel pipe is inspected. The experiment results show that the measurement technique can achieve total on-line measurement and it offers high precision and stability, which is of practical importance for the application.