An Ultra Wide Band (UWB) communication system with Dechirp and complex polyphase filter bank is proposed. The transmitted signal uses On-Off Keying (OOK) modulation and Chirp spread spectrum. The received signal goes through the Dechirp pulse compression, low-pass filtering and analog-digital conversion processes, and the subchannel division is carried out through the complex polyphase filter bank, followed by the subchannel selection and maximum ratio combination. Finally, the coarse timing synchronization, fine timing synchronization, SNR estimation and OOK demodulation through energy detection are realized. By means of the theoretical analysis and simulation experiments, the performance of this UWB communication system is evaluated over the AWGN channel, IEEE 802.15.3a CM1 and CM4 channel. The results show that this UWB communication system can achieve high processing gain and good anti-multipath capability, and that it is suitable for range extension communications.