Pulse Position Modulation is quite suitable for UWB. Existing methods for MPPM decoding are based on harddecision. As the information of the received signal has been deeply used by the soft decode technology, demodulation can be realized more perfectly. The soft decode technology of pulse position modulation was proposed. Formulas for soft-decision computaion have educed. Theoretic analysis and simulation results show that the proposed method achieves excellent performance than the hard decoding method, and that it is fit for UWB.