The invention relates to the technical field of a laser welding device, in particular to a laser welding online measurement compensating device, which comprises a numerical control three-axis laser welding machine tool connected with a laser welding joint with two numerical control rotary axes; the laser welding joint is connected with a camera device; the position, in which the camera device is mounted, is located in a position which is a distance L from the front end of the laser welding joint; the camera device is further provided with two numerical control rotary axes. The laser welding method and the device thereof can perform the online detection for the joint edge structure and the adjustment of the welding technique parameters on a real-time basis, so that the three-dimensional joint edge structure of a composite member has scene adaptability, that is, the welding technique parameter serves the automatic change adaption function of the welded seams on a real-time basis, thereby realizing the shape control and the performance control of the laser welding on a real-time basis.