The invention discloses a bimetal composite pipe girth welding method and belongs to the technical field of metal material welding. The bimetal composite pipe girth welding method comprises the following steps of carrying out seal welding of a stainless steel composite layer and a carbon steel base layer of a bimetal composite pipe to obtain seal welding lines, carrying out backing welding of the stainless steel composite layer to obtain a composite layer welding line, welding a stainless steel transition layer welding line on the composite layer welding line, carrying out isolated layer welding-line welding on the stainless steel transition layer welding line by a micro-carbon pure-iron welding material to obtain an isolated layer welding line so that the stainless steel composite layer is completely isolated from the carbon steel base layer, and carrying out carbon steel base layer welding to obtain a base layer welding line. The bimetal composite pipe girth welding method solves the problem that the existing bimetal composite pipe welding technology carries out welding of a stainless steel transition layer by a carbon steel or low-alloy steel welding material so that a hardenability layer is produced. The bimetal composite pipe girth welding method reduces a cost, improves welding line quality, guarantees corrosion resistance of a welding joint and safe and reliable operation of a pipe, and is an economic, effective, convenient and flexible composite-pipe welding technology.