Abstract: MP3 used MPEG-1 (Moving Pictures Experts Group) Layer III audio codec standard. Its compression ratio can be 1:10~1:12. As its high compression ratio and low Distortion, the on time decoding algorithm is more complex. Based on 16 bit fix point Digital Signal Processor, some optimization can be done in MP3 audio decoding. With the optimize algorithm such as fix point algorithm instead of floating point algorithm, sub-curve fitting, looking for table, the number of decoding instructions decrease to 1/20 per frame.