UPS can provide stable electrical energy for sensitive loads when severe grid disturbance happens. A neutral point clamped (NPC) three-level voltage source converter is used as the circuit topology for UPS. In this study, the discrete mathematical model of UPS is derived and a single voltage loop control strategy based on model predictive control is presented. The proposed method has a simple control structure and a fast response speed and omits the current inner loop and modulator. Furthermore, it can achieve ac voltage control and the balance of dc-side voltages. A three-level UPS simulation model is set up in PSCAD, and the efficacy of the proposed method is verified by means of simulation results under various types of loads.