A switchable multi-wavelength erbium-doped fiber laser is proposed. Based on the mode field mismatch principle, an in-fiber Mach-Zehnder interferometer (MZI) is fabricated by the non-core fiber combing with the polarization maintaining fiber. A stable and switchable multi-wavelength fiber laser can be realized by adjusting the curvature radius of MZI and controlling the polarization controller. The maximum wavelength spacing of the output multi-wavelength fiber laser is about 40.184 nm, and the side mode suppression ratio is larger than 50 dB. In the test of output fiber laser stability, the maximum wavelength variation of output wavelength is less than 0.06 nm in 1 h. The research results show that the proposed switchable multi-wavelength fiber laser has the advantages of a wide switchable wavelength range and a wide wavelength spacing range, a high SMSR, a stable output, and so on.