Describe a firm ware implementation strategy of on-line UPS digital control system which transplants an embedded operating system on DSP56F8356. The structure of single input and singe output on-line UPS firm ware is presented. The function of every task and interrupt service subroutine is deeply discussed. Program under this firmware structure has run on a 10KVA online UPS. The experiment result shows that each specification of UPS meets national standard and is leading the way of UPS industry.