The invention relates to the technical field of online weighing of the metallurgical industry, in particular to an online automatic weighing device of a continuous-casting continuous-rolling production line. The online automatic weighing device comprises a roller table, an electronic scale, a hydraulic transporter, carrying rails, position sensors and a controller. The online automatic weighing device is characterized in that a cut-off region is arranged on the middle part of the roller table, wherein the electronic scale is distributed in the cut-off region; the carrying rails are fixedly mounted at the two sides of the roller table; the hydraulic transporter is in blocking connection on the carrying rails; the electronic scale is electrically connected with the controller; the position sensors are distributed at the two sides of the roller table for detecting positions, on the roller table, of workpieces; and the position sensors are electrically connected with the controller. The online automatic weighing device has the substantial effect that: through an automatic weighing device integrated on the production line, weighing efficiency is improved, forklift workload is reduced, and workshop air pollution is reduced.