A method for adjusting clock frequency of a processing unit of a computer system includes calculating a busyness ratio of the processing unit according to a status signal provided by the processing unit, determining whether the busyness ratio is in a busyness ratio range, when the busyness ratio is not in the busyness ratio range, determining whether a calculation result generated according to a clock frequency of the processing unit and a frequency difference is in a frequency range, and when the calculation result is in the frequency range, adjusting the clock frequency of the processing unit according to the calculation result and outputting the adjusted clock frequency to a clock generator, wherein the busyness ratio range, the frequency range and the frequency difference are decided according to an operation state of a peripheral unit of the computer system.