The invention relates to an argon filling protection device for the welding back surface of a stainless steel or dissimilar steel branch pipe. The argon filling protection device is widely applicableto stainless steel or dissimilar steel welding in various industries. The upper end of a sliding rod is connected with a handle; the lower part of the sliding rod passes through a round pipe sliding rail; a threaded hole is formed in one end, close to the handle, of the round pipe sliding rail; the round pipe sliding rail is in threaded connection with a fastening screw; the round pipe sliding rail is connected with the two ends of a gas distribution pipe; the gas distribution pipe passes through a circular cover plate and is connected through welding; rectangular slot holes are formed uniformly in positions along 0-180 degrees of circumference by taking the center of the circle of the circular cover plate as the center,; a plurality of holes are uniformly drilled in the gas distribution pipe at the lower part of the circular cover plate along the circumference; an argon inlet pipe is welded on the gas distribution pipe at the upper part of the circular cover plate; the lower end of the sliding rod is connected with a bolt; and the bolt penetrates through a rubber piston pad and a gasket to be screwed with a nut. The argon filling protection device has the advantages of being applicable, economical, convenient and reliable, and the welding quality of the branch pipe is improved.