We proposed a compact and tunable multimode interferometer (MMI) based on an asymmetric wavy fiber (AMWF), which has axial offset, off-center taper waist, and micro-length. The fabrication process only contains non-axis pulling processes of single-mode fiber on two close positions. Theoretical qualitative analyses and experiments verify the tunable multimode propagation of the AMWF. Experimental results show a nonlinear wavelength response with increasing axis displacement from 0 to 120 μm. In the range of 0—10 μm, the sensitivity reaches the highest value of −1.33 nm/μm. Owing to its cost-effective, high-compact and tunable multimode propagation properties, the AMWF provides a promising platform for micro-nano photonic devices and optical sensing applications.