The invention discloses a flash welding forming method for a stainless steel large-section ring piece. The flash welding forming method comprises the steps that a stainless steel large-section ring blank with an opening is firstly fixed to a clamp, and powered on, the current is made to pass through the back from the left end to reach the right end to form a ring current, and preheating is carried out; the right end is driven to be close to the left end, and weak flashing is carried out; the feeding speed is improved, the current is adjusted, strong flashing is carried out, and a temperature field is formed; the length of a high-temperature mushy zone and the length of a high-temperature microstructure coarsening zone are calculated according to the temperature field, and therefore the upsetting distance is determined, and upsetting is carried out; pressure maintaining is carried out for a while. According to the method, the stable and reasonable temperature field can be formed, it can be guaranteed that all metal of the high-temperature mushy zone is ejected out of a base body in the upsetting process, full plastic deformation can happen to metal of the high-temperature microstructure coarsening zone, and the flash welding ring piece excellent in quality is obtained through reversion and recrystallization refined grains. The method is used for flash welding forming of the ring piece large in sectional area.