An　OVT　based　on　Mach-Zehnder　interferometer　is　proposed　in　this　paper,　which　helps　to　convert　the　electro-optic　phase　delay　into　the　displacement　of　the　interference　fringe.　And　then,　the　voltage　applied　to　the　electro-optic　crystal　is　obtained　by　positioning　the　displacement　of　the　fringe,　and　the　effectiveness　of　this　method　is　verified　by　experiments.　The　new　OVT　meets　the　requirements　of　0.5%　accuracy　class.　And　it　has　advantages　of　direct　linear　measurement,　independent　of　light　intensity　and　no　limit　by　half　wave　voltage　of　crystal　in　measurement　range.　Moreover　the　new　OVT　is　simple　and　easy　to　be　built　up.