A refractive index insensitive temperature sensor based on waist-enlarged few mode fiber (FMF) bitapers is presented. The first section of FMF is spliced between two single-mode fibers. In fusion process, the waist-enlarged FMF bitapers can be obtained by large current discharging repeatedly. The refractive index and temperature sensing mechanisms are analyzed. For the sensors with different sizes, the refractive index and temperature experiments have been performed. The results show that in the refractive index ranges of 1.335 0—1.346 6 and 1.348 2—1.419 3, the refractive index insensitivity is verified. In a temperature range of 31.9—90 °C, the sensor sensitivity can be up to 85.57 pm/°C. In addition, it has a compact structure. Therefore, the sensor can avoid the cross sensitivity for measuring the refractive index and temperature simultaneously.