A compact plasmonic refractive index sensor with ultra-high sensitivity has been proposed based on a metal-dielectric-metal waveguide coupled with atilted semi-ring cavity. A semi-ring cavity with a tilted angle of 70° to the horizontal direction is introduced above the MDM wave guideso that the symmetry characteristics of the resonator have been broken and more resonance modes can be generated. The transmission spectrum of a baffle-contained MDM waveguide coupled with a tilted semi-ring with a radius of 185 nm has been calculated numerically through the finite element method. Three transmission peaks with asymmetric Fano line-shape appear at the wavelengths of 594, 868 and 1 734 nm, respectively. The magnetic field distributions at these three wavelengths demonstrate that the third, second and first order resonance modes occurred in the semi-ring cavity lead to the three peaks in the transmission spectrum, which FR3, FR2 and FR1, respectively denote. Based on the coupling interference effect between the first three order narrow resonant modes and wide band reflected propagation modes in the MDM waveguide generated by the metal baffle, the formation mechanisms of triple Fano resonances leading to the three asymmetric transmission peaks have been clarified in detail. Meanwhile, the dependence of peak wavelengths on the refractive index of the dielectric materials has been calculated numerically, and the sensitivity associated with FR3, FR2, and FR1 are thus obtained as 550, 840 and 1 724 nm·RIU-1, respectively. In addition, an approximate analytical formula of the sensing sensitivity has been derived from the resonant condition of the semi-ring cavity, which displayed the linear dependence of the refractive index sensing sensitivity on the cavity length L and the reciprocal of the resonant order m. Obviously, the calculated three sensitivity values of the semi-ring cavity approximately satisfy this relationship. Finally, a split-ring with a split angle of π/2 has been obtained by extending the arc length of the semi-ring cavity without altering the radius of curvature so that the cavity length L has been lengthened 1.5-fold by increasing the center angle from π to 3π/2. Numerical results demonstrated that the sensing sensitivity of the three Fano peaks is increased to 821, 1 250 and 2 517 nm·RIU-1, respectively, which are 1.5 times the original values achieved in the semi-ring cavity scheme. Numerical results further verify the effectiveness of the approximate analytical formula, which provides a theoretical guideline for the design of compact high-sensitivity refractive index sensor.