The invention discloses a thin-walled barrel welding assisting tool. The thin-walled barrel welding assisting tool comprises an inner support assembly and an inner support lifting appliance; the innersupport assembly comprises a bottom platform, a supporting frame I, and a plurality of supporting plates I which are uniformly distributed in the circumferential direction of the supporting frame I;the supporting plates I are connected with drivers I arranged on the supporting frame I, and driven by the drivers I to synchronously move in the radial direction, and one side surfaces of the supporting plates I are closely attached to the inner wall surface of a barrel section; the supporting frame I is arranged on a vertical lifting mechanism; the inner support lifting appliance comprises a supporting frame II and a plurality of supporting plates II which are uniformly distributed in the circumferential direction of the supporting frame II; the supporting plates II are connected with drivers II arranged on the supporting frame II, and driven by the drivers II to synchronously move in the radial direction, and one side surfaces of the supporting plates II are closely attached to the inner wall surface of the barrel section; and the supporting frame II is arranged on a lifting moving mechanism so that the barrel section can be lifted and conveyed to the bottom platform. According to the thin-walled barrel welding assisting tool, the influence of the gravity factor on welding deformation can be eliminated, girth welding station automation is advantageously achieved, and the assembling quality and efficiency are improved.