The invention provides a method for forming a Ti3Al-based alloy thin-wall barrel. The method comprises the following steps of: (1) preparing a barrel blank by using a Ti3Al-based alloy plate with the thickness of between 1.5 and 4 millimeters at the temperature of between 650 and 700 DEG C by a hot rolling and bending method; (2) penetrating and welding a weld in the bus direction of the barrel blank by continuously inputting laser through CO2 so as to obtain a thin-wall barrel; (3) preserving the heat of the thin-wall barrel at the temperature of between 950 and 1,000 DEG C for 1 to 3 hours and cooling the thin-wall barrel along with a furnace; (4) calibrating the thin-wall barrel at the temperature of between 600 and 650 DEG C; and (5) preserving the heat of the thin-wall barrel at the temperature of between 900 and 1,000 DEG C in vacuum for 1 to 2 hours and cooling in the air. Due to the adoption of the method, cracks are effectively prevented in a forming process and deformation resistance is reduced; particularly, the intensity of the weld and a parent metal is lowered by thermal treatment after welding, plasticity is enhanced, and calibration after welding is facilitated and can be performed at a relatively low temperature; simultaneously, the intensity of the barrel is enhanced integrally through final thermal treatment.