Existing Photoelastic Modulator(PEM),whose optical path difference(OPD) is small,has strict requirements on the incident spot size and is poor in the aspect of light use efficiency under multiple reflections.What’s more,Photoelastic Modulator based Fourier transform spectrometer(PEM-FTS) spectral resolution is relatively poor.Because there are these disadvantages in the PEM,this paper presents a method of large optical path difference whose PEM is based on micro trapezoidal photoelastic crystals.By improving the structure of photoelastic crystal,the PEM becomes micro trapezoidal octagonal structure.And two transmission surfaces are changed slightly into a certain angle.Therefore,the PEM improved can not only increase the optical path difference of the PEM,but also have less requirements on the incident spot size.Firstly,a detailed analysis of the maximum modulation optical path difference was made in this paper.Secondly,the equation of maximum optical path difference was deduced under any angle and any position of incident light,vibration displacement and stress distribution of PEM are analyzed by the way of COMSOL Multiphysics 4.3a.Again,a method was analyzed to find the best angle of incidence,combining with maximum optical path difference and energy efficiency.Then the large OPD’s PEM is designed and processed,including two parts:photoelastic crystal and piezoelectric crystal.Moreover,ZnSe crystal is used as photoelastic crystal,and piezoelectric quartz crystal is used as piezoelectric crystal.With experiment analyzed by 632.8 nm He-Ne laser,the results show that under the same driving voltage,the optical path difference of the PEM improved is about 19.25 times bigger than the normal PEM,and the relative error is 1.3%.