A novel refractive index sensor based on etched multicore fiber Bragg gratings with temperature in-line compensation is proposed and experimentally demonstrated. By chemically etching the cladding of the multicore fiber, the six outer cores exhibit the sensitive responses to the surrounding refractive index change, with refractive index insensitive and temperature-sensitive central core inside of the multicore fiber. By using the a central Bragg wavelength in the multicore fiber as temperature compensators, the refractive index sensing can be in-line compensated. Moreover, the distribution of multiple outer cores enables the capability of avoiding the nonhomogeneous performance by averaging and balancing the read-out data. Theoretical analysis and experimental results demonstrate that this structure can easily discriminate the RI and temperature. The maximum sensitivity 42.83 nm/ RIU could be obtained at around 1.435 RIU, and the temperature sensitivity is 9.89 pm/°C. The proposed structure is able to in-line and in-situ determine refractive index and temperature simultaneously.