We present a novel switchable multi-wavelength Er-doped mode-locked fiber laser built on graded-index multimode fiber (GIMF)-photonic crystal fiber (PCF)-GIMF-based saturable absorber in this paper. The mode-locked pulses achieve a fundamental repetition rate of 18.72 MHz and signal-to-noise ratio of 48 dB. The GIMF-PCF-GIMF also acts as a Mach Zehnder interferometer to obtain single-, dual-, triple- and quadruple-wavelength switching mode-locked lasing output by changing the polarization state. The single-wavelength and dual-wavelength lasers generated by this device have the characteristic of wavelength tunability, and the wavelength tuning ranges are 25.29 nm and 7.26 nm, respectively. The new switchable multi-wavelength mode-locked fiber laser can simultaneously provide light sources for multiple channels in high-speed and large-capacity fiber communication systems, making the design of optical transmitters more compact and economical.