Abstract: According to equivalent circuit of the main welding circuit, a mathematic model was derived for maximum output power of theК-type alternative current rail flash butt welding machines. Based on the model, a high efficiency welding procedure was proposed, and the control program was designed. Experiments were conducted on aК900 rail flash welding machine by setting different welding currents at high voltage stages to determine the time at this stage. The experimental results indicate that welding time at high voltage stages is short when welding current at high voltage stage is close to the welding current corresponding to highest output power of the machine. Therefore, with the proposed procedure, the rail under welding can be heated fast, shortening the welding time and increase the productivity.