S-UPS power supply system is a high-reliability, small-sized, modular UPS power supply system designed specially for single power source equipment with high requirements for power supply reliability, such as PLC, server and other important loads. This system mainly consists of charging rectification module, inversion module, STS static transfer switchover module, monitoring system, and lithium and iron phosphate battery pack. The charging rectification module and the inversion module allow 1+1 parallel redundancy, and the modules are independent of each other without master/slave definition and with equal load sharing. The charging rectification module, the inversion module, and the STS static transfer switchover module allow hot plugging.