Since the energy of chirp waveform distributes evenly in time domain, chirp waveform has the advantage of low peak-to-average power ratio. In this paper, we first employ chirp function to generate the ultra-wideband (UWB) pulse for narrow-band interference (NBI) suppression, then the simplified characteristic function (CF) method is proposed to evaluate the NBI suppression performance of the direct sequence spread spectrum binary phase shift keying (DS-BPSK) chirp pulse based UWB (Chirp-UWB) system over multipath channels. Simulation results and theoretical analysis show that the proposed CF based performance analysis of Chirp-UWB system not only superiors to the traditional Gaussian pulse based UWB system, but also outperforms the conventional approximate analysis method in accuracy and complexity.