A D-class audio power amplifier including a microprocessor, a power amplifier, and a low pass filter is provided. The microprocessor is used to sample an input audio signal and output a pulse width modulation (PWM) clock signal. The power amplifier is used to amplify the power of the PWM clock signal and output a power-amplified signal. The low-pass filter is used to filter the high-frequency signal portion off the power-amplified signal and output an output audio signal. The method includes sampling the input audio signal to obtain a number of digital sampling values; obtaining corresponding duty cycles according to the digital sampling values and a reference voltage level and generating the PWM clock signal accordingly; power-amplifying the PWM clock signal to obtain the power-amplified signal; and filtering the high-frequency signal portion off the power-amplified signal to obtain the output audio signal.