The invention discloses an online measurement method for welding line characteristic point positions. The method comprises the following steps: a welding gun is positioned in an initial welding position; image information of welding line contours is initialized to obtain welding line initial positions; welding line images are acquired; welding line positions at present are predicted; the modelingof the welding line images in a time sequence direction is finished; characteristics of the welding line images are extracted; the tracking positioning of the welding line characteristics points is finally realized; and a motion trajectory of the welding gun is calculated. Meanwhile, the invention discloses a welding line trajectory automatic measurement system; and the system comprises an embedded development board, a laser visual sensor, a welding robot, a robot controller, mated welding equipment and a workpiece clamping worktable. The method can deal with the noise disturbance under non-structural welding environment, improves the robustness and the stability of a welding line tracking system, realizes automatic identification of welding line characteristic points, and greatly improvesthe automation degree and the production efficiency.