Abstract: A static transfer switch circuit for high reliability uninterruptible power supply system comprises a first switch unit which includes at least two switch sets connected in parallel and at least one second switch unit. Each of the switch sets has two controllable semiconductor switch elements connected in parallel reversely. The first switch unit includes one end connected to a bypass power source and another end connected to a load end. The second switch unit includes at least two switch sets or electromagnetic switches connected in series, and has one end connected to an output end of an uninterruptible power supply system main unit and another end connected to the load end. The static transfer switch circuit thus formed is simpler in structure and can enhance the reliability of static transfer switches used on a single machine or multi-machine uninterruptible power supply system.