A fiber-optic Mach-Zehnder interferometer (MZI) based on single-mode -multimode-thin-core-single-mode fiber structure is proposed and demonstrated for measuring refractive index and temperature simultaneously. The multimode fiber (MMF) and the thin core fiber-single mode fiber (TCF-SMF) spliced point serves as mode coupler. When the light is launched into the MMF through the lead-in SMF, at the MMF-TCF spliced point, the core mode and cladding modes are excited and propagate in the core and cladding region of the TCF respectively. The optical path difference can be produced between different modes propagating within the TCF. Then, at the TCF-SMF spliced point, the excited cladding modes coupled back into the core of lead-out SMF interfere with the TCF core mode. When the surrounding refractive index (SRI) and temperature change, the transmission spectrum of the sensor shifts. On this basis, the simultaneous measurement of SRI and temperature can be completed by monitoring the dips with different interference orders. The intermodal interference mainly occurs between the core mode LP01 and the cladding mode LP16 based on a fast Fourier transformation analysis for the transmission spectrum of the sensor. The sensitivities of the sensor response to the change of SRI and temperature are theoretically achieved. The SRI and temperature theoretical sensitivities of the interference dips at 1535 nm and 1545 nm are -55.90 nm/RIU, 0.0501 nm/℃, and -56.26 nm/RIU, 0.0505 nm/℃, respectively, where RIU is refractive index unit. The responses of the sensor are experimentally studied at the SRI range from 1.3449 to 1.3972 and temperature range from 20 ℃ to 90 ℃, and the sensitivities -53.03 nm/RIU, 0.0465 nm/℃ and -54.24 nm/RIU, 0.0542 nm/℃ for the two selected interference orders. The theoretical analysis of the sensor is in good agreement with that obtained in the experiment. This kind of sensor can offer attractive applications in biomedical sensing.