The invention relates to a default-phase protection circuit used for a three-phase alternating current power supply and a method thereof. The default-phase protection circuit comprises a first control module and a second control module. The first control module is powered by the first phase output and the second phase output of the three-phase power supply. The second control module is powered by the second phase output and the third phase output of the three-phase power supply. The first control module is connected with the second control module and is used for outputting a control signal to the second control module. The second control module is connected with a core power device of the three-phase alternating current power supply and is used for controlling the three-phase alternating current power supply to supply power externally according to the control signal and controlling the three-phase alternating current power supply not to supply power externally when the three-phase power supply loses a phase. According to the invention, the circuit is simplified, the weighting risk and production cost brought by the redundant design are lowered, the core power device of the welding power supply can be protected effectively and the reliability of the three-phase alternating current power supply can be improved.