A novel fiber sensor composed of a Multi mode fiber-Single mode fiber-Multi mode fiber(MMF-SMF-MMF,MSM) structure and a Fiber Bragg Grating(FBG) was proposed for simultaneous measurement of temperature and Refractive Index(RI) based on the interference between guided mode and cladding modes of a Single Mode Fiber(SMF). Due to the different responses of the MSM structure and FBG to temperature and RI variations, the proposed fiber sensor was used to measure the temperature and the RI simultaneously by utilizing a well-conditioned sensitivity matrix equation. The experimental measurement shows that the temperature sensitivity coefficients of the MSM structure and FBG are 0.055 2 nm/℃ and 0.015 8 nm/℃, respectively. The RI sensitivity coefficient of the MSM structure is 109.702 nm/RIU, whereas the FBG is insensitive to RI change. Furthermore, obtained sensing resolution is ±0.32 ℃ for the temperature and ±0.002 3 for the RI. Experiments also indicate that the temperature sensitivity coefficient of the MSM structure is five times over that of the SMS structure. Considering that the cladding mode nature of the SMF is more vulnerable to other parameters of the surrounding medium, the MSM structure can also be applied in other sensing fields.