The invention discloses a flash welding method and system. The method mainly includes the steps that a current sensor, a voltage sensor and a displacement sensor in flash welding equipment are used for collecting data of welding currents, welding voltages and feed displacement in the welding process correspondingly; the total energy consumption is calculated according to the welding currents, the welding voltages and flash time, and the energy loss is calculated according to the characteristics of a welding material and the data of the feed displacement; actual energy consumption is calculated according to the difference value of the total energy consumption and the energy loss; and when the actual energy consumption reaches a preset energy consumption desired value, an instruction of ending the flash welding heating stage and entering into the flash welding upsetting stage is sent to the flash welding equipment. By the adoption of the flash welding method and system, the energy loss in the heating process is taken into account, the effect that the welding material can be fully and evenly heated in the heating step is guaranteed, enough heat is stored, and after a temperature field with the mild temperature gradient is formed, upsetting at the next stage begins so that the welding quality can be made more stable.