A voltage adjustment device comprises a voltage detector and a signal emitter. The voltage detector electrically connects to an electrical device through a power rail and obtains a voltage detected value of the power rail. The signal emitter electrically connects to the voltage detector and is configured to electrically connect to a host and a power board. The signal emitter generates a power good signal and sends the power good signal to the host when the voltage detected value is larger than a baseline voltage value for the first time. After sending the power good signal, the signal emitter generates a voltage adjustment signal according to the voltage detected value and is configured to send the voltage adjustment signal to the power board for selectively adjusting a voltage provided by the power board.