A digital audio signal amplifier and a digital audio signal amplifying method therefor are provided. More particularly, a digital audio signal amplifier in which both safety and power efficiency are achieved by integrating a switching mode power supply and a digital audio amplifier into one, and a digital audio signal amplifying method appropriate to the amplifier are provided. The digital audio amplifier includes: a pulse modulation unit generating a pulse modulated audio signal by pulse-modulating an input audio signal; a switching unit switching a DC voltage based on the pulse modulated audio signal; an insulation transformer transforming the output of the switching unit and outputting the result of the transforming; and a low-pass filter obtaining an audio signal corresponding to the input audio signal by low-pass filtering the output of the insulation transformer, and outputting the output audio signal. By integrating a switching mode power supply and a digital audio amplifier into one, the digital audio amplifier satisfies the insulation requirement and at the same time increases power efficiency.