The modulation of resonance features in microcavities is important to applications in nanophotonics. Based on the asymmetric whispering-gallery modes (WGMs) in a plasmonic resonator, we theoretically studied the mode evolution in an asymmetric WGM plasmonic system. Exploiting the gap or nano-scatter in the plasmonic ring cavity, the symmetry of the system will be broken and the standing wave in the cavity will be tunable. Based on this asymmetric structure, the output coupling rate between the two cavity modes can also be tuned. Moreover, the proposed method could further be applied for sensing and detecting the position of defects in a WGM system.