To improve the anti-interference performance of UWB communication system, a new UWB communication system based on parallel combinatory spread spectrum is proposed after analyzing the characteristics of UWB and PCSS. The application of M-ary bi-orthogonal keying (MBOK) modulation in the new system is also discussed. In this paper, qualitative analysis is made to analyze the relationship between the system performance and band efficiency, and conclusion is got that the performance of PCSS-UWB communication system is related with the number of the transmission data in PCSS technology. Simulation shows that E_b/N_o is improved 9 dB in the system with MBOK modulation than that in the system with PPM modulation when BER is 10-4.