A Mach-Zehnder interferometer (MZI) for simultaneously measuring refractive index (RI) and temperature is proposed and verified in this paper. The sensor head is composed of thin core fiber (TCF) S-taper structure and spherical structure. By monitoring two interference dips, experimental results show that the RI sensitivities are -70.392 nm/RIU and ?60.08 nm/RIU in the RI range of 1.338 4—1.350 0, respectively. And the temperature sensitivities are 0.050 72 nm/°C and 0.071 7 nm/°C in the temperature range of 30—70 °C, respectively. The simultaneous measurementof the temperature and external RI is demonstrated based on the sensitive matrix. The sensor also has the advantages of low cost, simple structure and high sensitivity.