The frequency-selecting and sensing properties, in plasmonic system composed of waveguide and ring cavity with embedded metal bar, are numerically and theoretically demonstrated. Modulating the location of metal bar introduces the periodic variation of transmission spectra, which provides a new degree of freedom for controlling the resonant modes. In addition, the sensitivity and figure of merit reach 1 627.5 nm/RIU and 37.893 5, respectively. The results provide theoretical basis for the designing ultracompact optical switch, sensor and filter.