A battery discharging method for a computer system is disclosed. The battery discharging method is to detect a first detection value relative to a first status of a battery of the computer system and a second detection value relative to a second status of the battery, then to determine a clock adjustment parameter according to the first detection value and the second detection value, and at last to adjust an operation clock rate of a processor of the computer system according to the clock adjustment parameter. Therein, the statuses of the battery can be discharging temperature, discharging current, discharging voltage, residual capacity, or other statuses of the battery. Therefore, the invention can adjust the operation clock rate of the processor under the consideration to the statuses of the battery, so as to extend the discharging period of the battery and utilize the stored energy in the battery efficiently.