An energy saving system used in an electronic device is provided. The energy saving system includes a battery for supplying the electronic device with energy; a energy detector for measuring a energy level of the battery, and transmitting a battery energy saving control signal when the measured energy level is equal to or less than a predetermined energy level; and a control unit for changing a predetermined first time period into a predetermined second time period which is shorter than the predetermined first time period according to the battery energy saving control signal, and controlling the electronic device to enter a sleep mode state when an elapsed time of not receiving any operations on the electronic device reaches the predetermined second time period.