Abstract: A novel main circuit topology of ultrafast-convert complex high-frequency pulse square-wave variable-polarity arc welding power supply was designed and analyzed, which consisted of two parts.In the front part, the base current and pulse current were provided by two independent direct-current power supplies.In the latter part, direct current was transformed into series of high-frequency complex-pulse variable-polarity current, which crosses zero with no dead-time, by high-frequency pulse-forming circuit and fullbridge current-polarity transform circuit.The reliability and safety of the circuit system was effectively provided by assistant parallel-connection protecting circuit.The results of aluminum alloy variable polarity tungsten inert-gas welding show that the high-frequency complex-square-wave variable polarity current generated by novel circuit topology has fast rising and falling edge, which can satisfy the requirements of a novel high-strength and high-efficiency special variable polarity arc welding.