A novel plasmonic waveguide filter design based on three cascaded slot cavities is proposed. The cascaded nanocavities support a united resonant (UR) mode. Light is trapped in the middle nanocavity at telecommunication wavelength (1550 nm) when the UR mode exists. This phenomenon leads to the efficient transmittance and high Q factor of the plasmonic filter. The resonant wavelength and Q factor can be easily modulated by the cavity radii and the waveguide width.