An audio noise-free Uninterruptible Power Supply (UPS) with Zero-Voltage-Switching (ZVS) back-to-back (BTB) converter is proposed. Firstly, its ZVS sinusoidal pulse width modulation (SPWM) scheme is introduced. Using the proposed ZVS-SPWM scheme, all the switches in both the rectifier side and inverter side as well as the auxiliary switch can realize ZVS turn-on with only one commonly shared auxiliary circuit. Besides, the auxiliary switch only needs to operate once in each switching period, which can reduce the extra loss caused by the auxiliary circuit. The ZVS conditions with respect to both balanced and unbalanced load situations are given. A resonant parameter design guideline and a low stray inductance bus bar design are introduced. Finally, the proposed UPS system with ZVS BTB converter is verified on a 50 kVA experimental prototype.