A modal interferometer is experimentally demonstrated based on tapering a single-mode-multimode-single-mode (SMS) fiber structure heated by hydrogen flame. The interference fringe begins to form when tapering length is 19.8 mm, and becomes regular and clear when the tapering length is longer and the tapered waist diameter is smaller. Annealing process is undertaken to achieve a high extension ratio of approximately 17 dB with free spectral range of 1.5 nm when the tapering length is 33 mm and the tapered waist diameter is approximately 5 \mu m. The temperature and axial strain dependences of the tapered SMS structure are characterized, and the measured temperature and strain coefficients are 7 pm/oC and –9.536 pm/\mu \varepsilon, respectively.