The invention discloses a flash welding forming method of a constructional steel thin-wall ring member, which comprises the steps of: loading a D type ring blank with an opening in a welder, starting the welder to close the opening of the preheated ring blank, applying a 7-18V voltage between two poles of the welder to ensure that the left end face and the right end face of the opening of the ring blank are heated and molten through a flash current with the current density of 15A/mm<2>-220A/mm<2> to form a lintel, exploding to form a flash, wherein the flash burning loss speed of the ring blank is 3.6mm-7.2mm/s; driving the right end face of the ring blank to make an accelerated feeding movement towards the left end face at an initial speed of 2.5mm/s-4.0mm/s and an acceleration of 1.0mm/s<2>-2.0mm/s<2> so as to ensure that the distance between the left end face and the right end face is not more than 7mm; and after the flash is completed, driving a right straight side of the ring blank to move at a high speed towards a left straight side of the ring blank to ensure that the left end face and the right end face collide with each other under the action of an upsetting force of 9kN-15kN to form the flash welding thin-wall ring member, and keeping the pressure for 10s-30s under the action of the upsetting force. The method can ensure that the flash process is continuously and stably carried out, thereby improving the weld quality.