The invention discloses an argon filling back protection system for automatic pipeline welding. The argon filling back protection system comprises a circumferentially-segmented back argon filling protection mechanism (100), an argon source (400), a gas distribution electromagnetic valve group (200) and a gas distribution control module (300), wherein the circumferentially-segmented back argon filling protection mechanism (100) comprises a mechanism support (1) and a plurality of argon filling protection blocks (4), the mechanism support (1) is of a wheel type structure, and the argon filling protection blocks (4) are arrayed in the circumferential direction of the mechanism support (1) to form a circular shape. A gas chamber (45) is formed in each argon filling protection block (4). An argon interface (10) is formed in each argon filling protection block (4) and communicates with the corresponding gas chamber (45). A plurality of gas distribution holes (46) are formed in each argon filling protection block (4) and communicate with the corresponding gas chamber (45). By means of the argon filling back protection system, the construction efficiency and quality of stainless steel pipeline/bimetallic compound pipeline automatic welding can be improved, meanwhile argon can be saved, and construction cost is reduced.