A multifunctional inverter welding machine with the functions of DC gas tungsten arc welding, argon tungsten pulsed arc welding, manual arc welding and air plasma arc welding gather in integral whole is introduced in this work. In whose development, the theory of half-bridge series resonant inverter is used as the basis, while the fast SCR as the inverter. When in the situation of welding, full-wave rectification is used, with a working voltage of 36V; while in the situation of cutting, bridge rectifier is used, with a working voltage of 100V~125V. According to a large number of experiments and tests in the development process, the correctness of the mathematical modal of the fast SCR inverter is verified, and a new method of inverter circuit parameter design is obtained. By which, the theoretical basis is provided for the development of inverter welding machine.