Abstract: The working principle and process of the main circuit for 3-phase low frequency welder were discussed.After analyzing the conducting and shutting process of the six silicon-controlled rectifiers (SCR)with changing the pilot angle, the reliable phaseshift range of pilot angle was obtained, within which the main circuit can work steadily.Based on the above studies, the current-percentage-control mathematic model and voltage-compensation mathematic model were set up respectively, and the software flow of control system for main circuit was designed.A control system was made according to the control model and method in this study, and it was tested for controlling a 600 kW 3-phase low frequency welder.The welding experiment showed that, one of the SCRs will not shut normally when the current-percentage is up to 80%, which proved that the last conducted SCR is difficult to shut when the pilot angle is less than the power angle.