In order to abate the resonant frequency drift caused by crystal internal accumulated heat,we build the resonated dynamic thermal model of photoelastic modulated interferometer.By analyzing the impedance property of the cascade resonance model,we obtain the relation between interferometer′s heat dissipation power,feedback current and the phase of the driving voltage.Eventually,we establish the formula that describe the relationship between the temperature change rate and time,and the relationship between the resonant frequency change rate and time.The state equation of frequency stability is obtained.Analysis shows that in order to get the maximum optical path difference for the photoelastic modulator，we should adopt phase locked tracking method to keep drive frequency consistent with resonate frequency.