The invention discloses a built-in auxiliary welding device for tubular busbars. The built-in auxiliary welding device comprises a supporting device, a circumferential driving device and a built-in auxiliary butt joint device, wherein the supporting device comprises V-shaped clamping grooves, circumferential rolling wheels and a longitudinal moving auxiliary device; the circumferential rolling wheels are arranged on the two sides of the V-shaped clamping grooves; the circumferential driving device comprises a driving belt, a driving belt bracket, a driving motor and an adjusting cylinder; the built-in auxiliary butt joint device comprises a straight cross bar; radial supporting devices are symmetrically arranged at the two ends of the straight cross bar; and each radial supporting device comprises two mutually separated annular bosses, annular inflatable cushions and arc-shaped plates. According to the built-in auxiliary welding device, the arc-shaped plates are tightly propped against the inner side walls of the tubular busbars under the action of the annular inflatable cushions, so that the two tubular busbars can be stably connected at the butt joint part through utilizing the built-in auxiliary butt joint device; and during hoop welding, the tubular busbars are prevented from deviating at the butt joint part, and the straightness of the tubular busbars at the butt joint part is guaranteed.