The invention discloses a method and a device for welding a tube type heat exchanger by using laser scanning. Under the control of a computer program and according to tube parameters of the tube type heat exchanger, data such as laser parameters, dynamic focusing scanning control parameters, and the like is determined, and the heat exchanger components are welded and processed by adopting a laser beam deflection scanning method. The device for realizing the method comprises a high power laser, a collimated beam expander, a diaphragm with water cooling, a dynamic focusing system, an XY beam positioning scanner, a CCD photographing system, a Z-axis liftable workbench and a computer control system. The invention has the advantages that after the welded workpiece is fixed, only the focused laser beam is arranged in a determined three-dimensional processing space under the control of the computer program, and the tube type heat exchanger is subjected to non-contact type welding along a welding seam locus in a dynamic focusing laser beam scanning welding method. The invention overcomes the difficulty in the manufacturing of the tube type heat exchanger and realizes the precision manufacturing of a stainless steel ultrathin-wall tube type miniature heat exchanger.