The intensity-dependent loss output characteristic of the symmetric nonlinear optical loop mirror (NOLM) can be induced by the nonlinear polarization rotation effect. The application of symmetric NOLM as the intensity equalizer to suppress the mode competition in multiwavelength eribum-doped fiber laser is studied. The influences of both input polarization and birefringence on the output characteristic are analysed. The polarization conditions in this laser cavity are analysed. A multiwavelength eribum-doped fiber laser with this device is porposed and demonstrated successfully. In the L-band 13 lasing wavelengths,with 1nm spacing in 3 dB range,has been obtained,single-wave length power fluctuation of each laser line is less than 0.6 dB within 0.5 h. Both the laser line number and its multiwavlength operation spectral region can be controlled by adjusting the intensity-dependent loss in the cavity.