Abstract: Focusing on the extruded Q345 square steel tube, the edge/corner mechanical property-changing regularities before and after low-temperature stress-relief annealing process are compared and analyzed. As a result, it is identified that the said tube's corner yield strength ratio can be significantly reduced owing to 250~300℃ low-temperature stress-relief annealing treatment so as to get the overall service performance of the material improved. Some new ideas concerning improvement of the manufacturing technology of the square and rectangular tubes are put forward here in the paper.