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A novel optofluidically tunable Thulium-doped fiber laser (TDFL) based on a multimode interference (MMI) fiber filter is experimentally demonstrated with a wide tuning range from 1813.52 to 1858.70 nm. The wavelength tuning of the TDFL is achieved by employing an MMI fiber filter which is formed by splicing a segment of a special no-core fiber that is an all silica fiber without fiber core to single mode fibers. The no-core fiber with a large diameter of 200 μm is gradually vertically covered by refractive index matching liquid, which leads to a wavelength tuning of the transmission peak of the MMI fiber filter. The relationship between the refractive index of the refractive index matching liquid and the peak wavelength shift of the MMI fiber filter is also discussed. Using the MMI fiber filter, a Thulium-doped fiber laser with a tuning range of 45.18 nm, a side-mode suppression ratio better than 40 dB, and a 3 dB bandwidth less than 0.16 nm is demonstrated.