The invention discloses a narrow-gap plate pipe electric resistance welding device which comprises a power loading mechanism, wherein the piston rod of the power loading mechanism is hinged to one end of a crank arm; an upper pin roller electrode is mounted at the tip of the other end of the crank arm; a lower pin roller electrode is arranged below a position matched with the upper pin roller electrode and is mounted on a lower cross beam; the crank arm is hinged to the lower cross beam through an insulating pin shaft; the lower cross beam and the power loading mechanism are mounted on a device platform to form the electric resistance welding device; the electric resistance welding device is mounted on a numerical control hydraulic machine to realize movement in the up-and-down direction, and realize movement in the front-and-back direction through a transmission mechanism. According to the invention, the problems that a welding seam is low in strength and is likely to be corroded at the end part of a plate bundle during the manufacturing process of a traditional welding plate heat exchanger are solved.