The principle of polarized light interference is quantitatively demonstrated using experiment of

crystal electro-optic effect. With the help of index ellipsoid and small angle approximation，it is summarized that

inherent order of stripe under convergent incident light is depended on position of stripe and driving voltage loaded

on crystal. Extracting data from interference fringes，the linear relationship between the interference order and the

square of the dark pattern position is verified，in different driving voltages，the correlation coefficients are all higher

than 0.99; the electro-optic coefficient of crystal is measured by using the relationship between driving voltage and

interference order，the error of the result is about 1.45% compared to accepted method. The experimental design

can deepen the understanding of the theoretical knowledge of polarized light interference for junior undergraduates

and enrich the experimental content.