The uninterruptible power supply system plays a crucial role in ensuring the continuity of power supply for critical loads. However, during situations such as power outage, restoration, or bypass switching, the traditional mode switching mechanism of the uninterruptible power supply often leads to voltage and frequency fluctuations, and even temporary power interruptions, which may cause potential damage to sensitive electronic equipment such as servers, medical devices, and precision instruments, affecting their stable operation and data integrity. To address this technical challenge, this paper proposes a new smooth mode switching strategy for uninterruptible power supply systems. This research mainly focuses on online uninterruptible power supply systems and studies the key technologies for achieving seamless and smooth mode switching in the event of power supply fluctuations. A method for feedforward control of the uninterruptible power supply output voltage is proposed, enabling the output voltage of the uninterruptible power supply to quickly return to stability after power supply fluctuations. A phase-locked control method for the uninterruptible power supply output voltage is also proposed to ensure the stability of the uninterruptible power supply during power supply fluctuations. Experimental verification using actual hardware prototypes shows that the proposed smooth switching strategy can control the amplitude of voltage fluctuations during the switching process within 2%, the frequency deviation within 0.5 Hz, and the switching time to less than 10 milliseconds. Compared with traditional switching methods, our approach significantly reduces the interference during the switching process, ensuring the normal operation of connected devices.