Abstract: During normal operation, the UPS should provide the load power continuity and reliability. Because of the high integration of the electronic equipment, the output of the UPS shutdown while the control board fails. In this paper, the failure of UPS output shutdown is analyzed, and two kind of improvement of UPS main control board are provided based on the single fault criterion, which are forced static bypass switch and independent static bypass switch. The feasibility of the scheme has been verified by the test, and the reliability of UPS has been improved.