The　half-wave　voltage　was　obtained　by　measuring　the　phase　retardation　of　the　electro-optic　crystal　under　the　applied　external　voltage.　Based　on　polarization　interference　of　birefringent　crystal,　phase　shift　of　polarized　light　is　transferred　to　the　movement　of　interference　fringes,　and　the　linear　accurate　measurement　is　complemented　by　locating　dark　fringes　position.　The　experimental　results　indicate　that　the　accuracy　of　phase　retardation　of　lithium　noibate　crystal　is　4.4×10-3　rad,　the　half-wave　voltage　that　LN　crystal　is　measured　as　480.0　V　with　experimental　error　of　0.10%.　Compared　with　the　experimental　error　of　0.96%　that　half-wave　voltage　is　measured　by　extremum　method,　the　measuring　error　of　polarization　interferometry　method　is　reduced　obviously.　The　optical　structure　of　polarization　interferometry　method　is　simple　with　high　measuring　accuracy,　the　measurement　result　is　insensitive　to　the　power　fluctuation　of　the　light　source　and　the　measurement　range　of　phase　retardation　of　electro-optic　crystal　can　be　largely　expanded.　©,　2015,　Chinese　Optical　Society.　All　right　reserved.