Friction stir welding head capable of increasing weld penetration

The invention discloses a friction stir welding head capable of increasing weld penetration. The friction stir welding head comprises a stirring pin, a shaft shoulder and a clamping part which are sequentially connected, wherein the stirring pin is of a frustum structure with threads on side surfaces and three uniformly distributed cut planes; and a root thread end structure of the stirring pin is included in a forward side edge in a rotation direction of a stirring head in any cut plane on the side surface of the stirring pin; and the end part planes of the stirring pin are uniformly distributed in three circular-arc spiral grooves or circular-arc straight grooves. The friction stir welding head increases a friction area between the end parts of the stirring pin and welding seam metal, strengthens stirring ability of a root welding seam material, improves the temperature of root welding seam metal, promotes plasticizing and flowing of a root welding seam material, increases weld penetration and adaptation to