In this paper, a novel high-sensitive mid-infrared photonic crystal-based slotted-waveguide coupled-cavity sensor to behave as a refractive index sensing device is proposed at mid-infrared wavelength of 3.9 µm. We determine the sensitivity of our sensor by detecting the shift in the resonance wavelength as a function of the refractive index variations in the region around the cavity. Comparison between mid-infrared photonic crystal-based slotted-waveguide coupled-cavity with mid-infrared photonic crystal-based slotted-waveguide shows a higher sensitivity to refractive index changes. The sensitivity can be improved from 938 nm/per refractive index unit (RIU) to 1161 nm/RIU within the range of n = 1 - 1.05 with an increment of 0.01 RIU in the wavelength range of 3.3651 µm to 4.1198 µm by creating a microcavity within the proposed structure, calculated quality factor (Q-factor) of 1.0821 x 107 giving a sensor figure of merit (FOM) up to 2.917 x 106, and a low detection limit of 3.9 × 10-6 RIU. Furthermore, an overall sensitivity is calculated to be around S = 1343.2 nm/RIU for the case of higher refractive indices of analytes within the range of n = 1 - 1.2 with an increment of 0.05 RIU. The described work and the achieved results by performing 2D-finite-difference time-domain (2D-FDTD) simulations confirm the capability to realize a commercially viable miniaturized and highly sensitive mid-infrared photonic crystal based slotted-waveguide coupled cavity sensor.