Monitoring refractive index and axial strain is crucial in engineering applications. To achieve a large range sensor for the measurement of dual parameters of refractive index and strain, we propose a fiber sensor based on a tapered thin-core fiber (TCF). A compact all-fiber Mach-Zehnder interferometer (MZI) is fabricated by fusing a 2 cm length of TCF between two standard single-mode fibers (SMFs), charging and tapering it in the center. In our experiment, we extend the measurement range of the refractive index by using propylene glycol solution instead of sodium chloride solution. The results show that the highest sensitivity of the refractive index is -20.95 nm/RIU in the range of 1.351 1—1.380 3 and can reach -294.32 nm/RIU in the range of 1.384 3—1.446 6. In the strain range of 0—272 7 με, it exhibits a sensitivity of -0.6 pm/με, indicating that the structure maintains strong robustness even with a large refractive index range and high sensitivity. The device is simple to manufacture, compact, and has application value in refractive index and strain measurement.