A plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguide-coupled structure is proposed and demonstrated in this paper. The physical mechanism of the device is deduced, and the finite difference time domain (FDTD) method is employed to simulate and study its index sensing characteristics. Both analytic and simulated results show that the resonant wavelength of the sensor has a linear relationship with the refractive index of material under sensing. Based on the relationship, the refractive index of the material can be obtained from the detection of the resonant wavelength. The results show that the sensitivity of the sensor can exceed 1600 nm/RIU, and it can be used in chemical and biological detections.