Abstract: The embedded multi-task real-time operating system is a method to improve the dynamic characteristic of arc welding inverter.A digital arc welding power supply was designed,ARM(advanced reduced instruction set computer machine) as controller and μC/OS-Ⅱ as operating system.Welding processing was divided into 15 tasks with the principle that major task enjoyed more priority.Two semaphores were introduced to avoid simultaneous-sharing on the same source from several tasks.To accelerate the major task processing speed,data would be transferred to the minor tasks in the form of message box.