Longitudinal seam welding machine series are mainly used for welding longitudinal seam of thin sheet. It adopts cantilever type, and it is mainly composed of frame, fingers-type clamping unit, water-cooling mandrel, travelling carriage, crossing slider, wire feeder and torch holder. Fingers-type clamping unit owns special push from air cell, pressure of air cell is adjusted between 0 to 7kg/cm2, and the distance of two banks of fingers between 5 to 20mm to satisfy various welding seam width; Water-cooling mandrel is designed according the work pieces' diameter and length to ensure the strength. The top of mandrel is inserted water-cooling insert with groove for heat-sink cooling. It is adjustable in mandrel height to handle various material thicknesses. Beam is fitted with travel carriage which is moved straightly by track and driving unit, variable speed.