Abstract: Staying with manual measurement of circumference of the HSAW pipe are such shortcomings as slow measurement operation and big accuracy errors.Accordingly an on-line measuring system is presented here in the paper.Elaborated are the main aspects of the said system, including the measurement principle, the structure, the ways to realize the functions, and the on-the-site operation status.Also conducted is an analysis of the reasons for the measurement errors of the system.With the non-contact measurement method, the system is capable of performing on-line highprecision circumference measurement of the large-sized HSAW pipe under high-welding speed operation condition, and also indicating the circumference variation trend.The system has been up to the requirements for on-line measurement of steel tube during actual production process in terms of measurement accuracy and measurement speed, having brought about realization of real-time on-line circumference measurement at the formation station of the HSAW pipe.