A power-management method is provided, and the power-management method includes setting a central processing unit in a first low-power state when receiving a second low-power state request requiring the central processing unit to enter the second low-power state, obtaining first idle periods of the peripheral modules respectively to determine a second idle period according to the first idle periods of the peripheral modules, determining whether the peripheral modules have not sent a data-access request during the second idle period, setting the central processing unit in the second low-power state when the peripheral modules have not sent the data-access request during the second idle period, wherein each first idle period is an interval period between two data transmissions of each peripheral module.