An automatic start-up circuit for use in an uninterruptible power supply apparatus is provided. The uninterruptible power supply apparatus includes an energy storage unit and a switching circuit. The automatic start-up circuit includes an operating status detecting circuit and a secondary switch driving circuit. The operating status detecting circuit is connected to an energy storage unit connecting node for detecting an operating status of the energy storage unit through the energy storage unit connecting node, and generating an start-up control signal according to the operating status of the energy storage unit. The secondary switch driving circuit is connected to the operating status detecting circuit, a control terminal of the switching circuit and a common terminal for determining whether the switching circuit is automatically driven to be conducted to start up the uninterruptible power supply apparatus according to the start-up control signal.