Based on the structure of fiber loop mirror (FLM) and the theoretical a nalysis of its filter characteristics,the FLM is used to flat the gain of Er 3＋/Yb3＋ co-doped double-cladding fiber amplifier (EYDFA).The fi lter characteristics of FLM with one and two sections of polarization maintainin g fiber (PMF) are analyzed in the experiment under different lengths of PMF a nd different st ates of polarization controller.On that basis,we also ana lyze how to set the length and select the numbers of PMF for the FLM properly.In the experiment,we use the FLM with two sections of PMF t o flat the gain of the amplifier,which makes the two peaks of the ASE spectrum flat rela tive ly.With this method,the gain fluctuation of the ASE spectrum is reduced to ±0. 75dB from ±7.5dB in the main unflat wavelength range of 1535－1547nm.And the ou tput signal of broadband source is input into the amplification system as the si gnal light,and the gain fluctuation of the amplifier is reduced to ±1.2dB from ±4.5dB in the same wavelength range of 1535－1547nm.