A subwavelength plasmonic waveguide filter with symmetric grating distribution is proposed by using the metal-insulator-metal (MIM) structure. By cascading two gratings with different widths in a period, we can increase the number of pass bands, as well as the transmittance of each band up to ~70% compared with gratings with the same width. Such a device can find applications in various optical systems as wavelength demultiplexing component.