Abstract: The control system with TP-801 microcomputer for the SCR-inverter arc welding power source is discussed in this paper. It contains; (1) principle of the control, function of TP-801 in the system, A/D and D/A interface circuit; (2) selection of the controlling and controlled objects; (3) design of the system softwares; (4) ways of obtaining and modulating the constant-current and constant-voltage external characteristics of the SCR-inverter arc welding power source.