A widely tunable linear-cavity double-cladding Ytterbium-doped fiber laser based on multimode interference (MMI) effect is reported. The laser cavity consists mainly of MMI fiber reflecting elements, which is composed of single-mode fiber, multi-mode fiber and broadband metal reflecting mirror, double-cladding Ytterbium-doped large-mode-area (LMA) fiber, broadband dielectric total reflecting mirror and 3 dB output coupler. The wavelength tuning of fiber lasers is realized by moving the mirror in MMI fiber reflecting elements, which is changing the reflection peak wavelength. A wavelength tuning range of about 32 nm ranging from 1038.82 nm to 1071.06 nm is experimentally demonstrated by utilizing the optical spectral analyzer, with the 3 dB linewidth of 0.2 nm and the signal-to-noise ratio of roughly 40 dB. The average output optical power of 430 mW is obtained with 1.5 W of the launched pump power at 974 nm, and the slope efficiency is about 31%.