For high-speed UWB systems working in dense multipath environments, Inter Pulse Interference (IPI) is a primary obstacle to achieving good performance. The impact of the pulse waveform on the IPI is analysed. The analysis in this paper shows that the IPI becomes the lowest when the spectrum of the waveform is rectangular in pass band. For practical considerations, the spectrum of the waveform is restricted by the spectral mask and the distribution of energy of pulse should be concentrated in a time slot, so a compromise should be made between these issues in waveform design. The optimal pulse waveform restraining the IPI, which is meanwhile compliant with the FCC spectral mask, is designed as an example.