A sub-wavelength surface plasmon polariton wavelength filter which consists of an L shape cavity and a rectangle cavity coupled to a straight metal-dielectric-metal(MDM) waveguide was proposed and investigated numerically. The simulation results show that the real and imagine part of effective index changed rapidly at short wavelength and reached a constant at long wavelength 800～2000 nm. The propagation distance increased with the increasing of wavelength and waveguide width. When the waveguide width is over 50 nm it reaches 4～9 μm which can meet the photonic device. The L shape cavity and rectangle shape cavity side coupling reached a similar result as if their length equal to each other,it showed that the forbidden band only has relation with cavity length and it has a reference meaning for development of integrated photonic device.