In the present paper a phase controllable waveplate model was applied to the analysis of polarization maintaining fiber. Under the temperatures of 24 ℃ and 26.8 ℃ there is no residual amplitude modulation (RAM) existing in frequency modulation spectroscopy (FMS) when performing the measurement of fiber temperature dependence on the RAM. However, the temperature setting can not reduce the background long term drift and a servo feedback suggested by N. C. Wong and J. L. Hall can be used to actively reduce the RAM. The error signal for feedback is from the dispersion background signal of FMS without gas sample. The variation of RAM induced by the temperature changing of PM fiber is the main reason for the long term background drift of dispersion signal of FMS.