A refractive index sensor based on single-mode fiber (SMF) four-core fiber (FCF) and thin-core fiber (TCF) is proposed, forming a SMF-FCF-TCF Michelson interference structure. The optical fibers are spliced by direct splicing. Because of the mismatch of the diameter of the optical fibers’ cores, light excitation and coupling will be induced at the splicing part. The end face of the TCF is coated with a layer of silver film and protected with ultraviolet curing glue to enhance the reflectivity of the light at the end face. The four-core fiber is used as a coupler in the sensing structure, which excites more light into the cladding of the TCF, improving the sensitivity of the sensor. The refractive index and temperature sensing characteristics of the sensor were investigated experimentally. The experimental results show that the sensitivity in the refractive index range of 1.3333 to 1.3794 is 137.317 nm/RIU, the linearity is 0.999, and the temperature has little effect on the sensor. The sensing structure has a simple welding method and has certain application prospects in the field of refractive index measurement.