Abstract: To accurately obtain the true constitutive relationship of each material area in the girth welds of high-strength pipeline steel. Definitely, two kinds of notched round bar specimens were obtained by setting notches on the base metal and in the weld area of X80 steel respectively, and the tensile tests were carried out on the specimens, with the real-time images of specimen notches captured by the high-speed cameras during the tensile process. Then, based on the change law of gray value at the notch edge, an automatic measurement method, i.e. the minimum diameter tracing method, was proposed to measure the minimum sectional diameter of the notch area in real time. In addition, the true constitutive relationship between the base metal and the weld material can be obtained based on the minimum diameter tracing method and the modifier formula, and by fitting the curve obtained with the power law hardening model, the constitutive relationship parameters of corresponding materials can be gained. Finally, the test results were verified with the finite element method. The results show that both of the test method and the results are reliable and effective. The studied test method and the process are of great practical significance for obtaining the constitutive relationship between the base metal and the weld area and realizing the accurate test of the mechanical performance of the pipeline girth welds.