We report an all-fiber switchable dual-wavelength mode-locked laser using a Sagnac loop mirror. In this laser, the Sagnac loop mirror acts as a tunable periodic filter, in which the wavelength spacing is determined by the length of the polarization maintaining fiber. With appropriate pump power, the mode-locked fiber laser can operate in both the tunable single-wavelength mode locking regime and the dual-wavelength mode locking regime by properly setting the states of the polarization controllers. In the single-wavelength mode locking regime, the laser achieves continuously tunable wavelength range from ∼1551 nm to ∼1562 nm. In the dual-wavelength mode locking regime, the laser can emit two stable asynchronous pulse trains with the central wavelengths of 1561.4 nm and 1551.1 nm, respectively. Due to the net negative dispersion of the laser cavity, the mode-locked pulse trains with different central wavelengths have different group velocities leading to a repetition rate difference of ∼530 Hz.