In order to improve the accuracy and stability of the rebuilt spectrums,it is necessary that stability analysis and nicety measuring of the maximum optical path difference of interferograms in the photo-elastic modulator Fourier transform spectrometers(PEM-FTS).The maximum optical difference of interferograms is uncertain parameter,and it is relate to the resonant state,characteristic of frequency-thermal drift and driving voltage of PEM.Therefore,based on the principle of photo-elastic modulator Fourier transform interferometer,the model of the freguency-thermal drift is built,and the variety of the maximum optical path difference is analyzed;A measuring method of the maximum optical path difference is put forward,which is zero-crossing counting of laser’s interference signal when the driving signal of PEM is as the standard.In the method the dual channel high-speed comparator and FPGA are used to transform sine wave to square wave,to realize zero-crossing trigger counting and errors compensation.On the condition that the 670.8 nm laser is as the power source to produce the reference interferograms by the PEM interferometer,the 77.471 μm maximum optical path difference could be measured by the zero-crossing counting.the measuring errors is less than 0.167 nm,the rebuilt spectral peak wavelength errors of the infrared blackbody is less than 2 nm.the result is content with PEM-FTS.