We demonstrate a thulium-doped mode-locked fiber laser with ultra-broadband wavelength tunability for micro-strain sensing based on the multimode interference (MMI) effect in single-mode-multimode-single-mode (SMS) fiber configuration. The homemade SMS device with high performance is fusion spliced in the laser cavity, and the developed dispersion precisely managed the all-fiber structured mode-locked picosecond laser with a record-breaking wavelength tuning range from 1976 to 1916 nm while exerting axial strain on this SMS device. We experimentally explored the regularity between the strain and the central-wavelength shift of the mode-locked pulse, and for the first time to the best of the authors' knowledge, achieved the precise in-line axial strain measurement from 0 to 5385 µɛ by using the tunable ultrafast-laser-based sensor, and sensitivity is up to -11.5 pm/µɛ. With high compactness and durability, this sensor has advantages in real-time dynamic measurement over other passive devices, thus will undoubtedly find various application scenarios.