An outdoor monitoring system includes a camera, a direct-current adapter, a solar cell unit and a power management unit. The power management unit includes a controller, a rechargeable power source, a first voltage detection circuit, and a second voltage detection circuit. The first voltage detection circuit detects a charging voltage of the solar cell unit for charging the rechargeable power source. The second voltage detection circuit detects an output voltage of the rechargeable power source applied to the camera. The controller selectively connects the rechargeable power source or the direct-current adapter to the camera according to the output voltage and connects or disconnects the rechargeable power source to the solar cell unit according the output voltage and the charging voltage.