Abstract: Through analysing the commutating circuit work waves of half-bridge thyristor arc welding inverter,a RLCU-type equivalent circuit of capacitor discharge is put forward. With the nonzero initial condition of current and voltage, the second order linear ordinary nonhomogeneuos differential equation of capacitor voltage is built up,and its solution completelly according with the praactically measured waves is derived. With front and back edges of the current pulse, the theory of current shifting process in constent magnetical condition is introduced, and two equivalent circuits corresponding to both of the edges are delivered. Inverse voltage forming of the capacitor and its varying rulse in the current back edge period are discussed. Thereby the work principle of this commutating circuit is understood clearly, and an overall and correct foundation for engineering design of the inverter is found.