The resonance properties of surface plasmon in the IMM/dielectric/IMM waveguide are theoretically studied by using finite-difference time-domain technique, where the claddings are indefinite metamaterials (IMMs). From the dispersion relation, it is found that the IMM/dielectric/IMM waveguide supports TE polarized surface plasmon if IMM is always-cutoff with negative permeability. For an IMM/dielectric/IMM waveguide with a finite length, a subwavelength plasmon microcavity can be formed by Fabry-Perot resonance. At the resonant frequency, the magnetic field is maximized at the dielectric core entrance and exit, the electromagnetic energy is strongly concentrated around the dielectric core. When an artificial magnetic resonator is put at the core entrance and the resonance frequency is tuned to the plasmon microcavity mode, Rabi splitting can appear because of the strong coupling between this resonator and the cavity mode.