Ultra-narrow-band communications system can greatly improve the spectrum utilization efficiency, therefore it will ease the tension of spectrum. Unfortunately, arguments from academia still focus on its principle as well as the realization. UNB transmission system based on the FIR filter has been explored in this paper. The characteristic of UNB signal after the FIR filter was analyzed. Then information recovery has been performed using maximum likelihood receiver. The rationality of using the energy of continuous spectral side-lobe has also been discussed. As simulation has showed, if the bandwidth of RF filter is equal to the signal's NYQUIST'S bandwidth, the BER performance of UNB transmission system is only 1dB poorer than BPSK when BER dropped to 10-5, while the spectrum utilization efficiency can exceed 100bits/s/Hz.