We demonstrate a multiwavelength Tm-doped fiber laser employing polarization-maintaining undoped fiber (PMF) based on nonlinear polarization rotation. The PMF and a polarization-sensitive isolator between two polarization controllers are used to induce the intensity-dependent loss and frequency-dependent loss to alleviate mode competition caused by homogeneous gain broadening in an active fiber. An up to four wavelengths laser operating in a cavity is observed without the PMF. After the PMF is inserted into the cavity, stable dual-wavelength operation is achieved. The output had an average power distribution. The spectral fluctuation at each wavelength is smaller than 0.9 dB.