In this paper,a method for measuring the complex tensor components of electro optic constants and the thickness change of the poled dye doped polymer films using a Mach Zehnder interferometer is introdused.With this method,the complex electro optic constants of poled disperse red 1(DR1) dye molecules doped in PMMA polymer films which with symmetrical sandwich structure at 632.8 nm were determined.The modulated electro optic signals were given by accounting with the thickness change due to the modulated applied voltage.It is observed that the real and imagine parts of the complex tensor components have different contributions to the electro optical effects.The results show a large influence upon to the measurements of the electro optic constants especially when the modulated frequency was near to the resonance frequency of the sample.The resonance frequency strongly depended on the weight of the glass substrate and could be determined using a suppressed vibration model.