An improved double-mode surface acoustic wave(DMS) structure is used to design an extreme narrow band SAW filter at 1.5 GHz. The reflectors at both ends of the DMS structure adopt a distributed multi-period weighting structure,which can eliminate a variety of acoustic reflection modes propagating on the acoustic channel. A reflector is added between two interdigital transducers(IDT) in DMS structure to flexibly design the stopband suppression and bandwidth index of the filter.The results show that the developed extreme narrow band SAW filter has a center frequency of 1.5 GHz, a measured bandwidth of 878.75 kHz,insertion loss of 5.8 dB, and stop-band suppression of 45 dB.