A switchable fiber laser based on Mach-Zehnder interferometer (MZI) and photonic crystal fiber (PCF) is demonstrated. A section of PCF in the fiber laser is used to induce four-wave mixings (FWM) effect and MZI acts as a comb filter. Experimental results show that stable single-wavelength, dual-wavelength and triple-wavelength operations can be obtained with the adjustment of the polarization controller. The power of two peaks of the dual-wavelength laser is almost the same under room temperature with an output power fluctuation less than 0.6dB. The advantages of the proposed fiber laser include quite stable wavelength and narrow linewidth owing to the contributions of FWM in the PCF.