This paper presents an optic-electronic voltage transformer insulated by SF6, Which is applied in 500KV (line voltage) power system. The voltage sensor is based on converse piezoelectric effect in a quartz crystal. High voltage is exerted on the quartz crystal after being divided by cylinder capacitor divider, piezoelectric deformation is sensed by elliptical-core dual-mode fiber around the surface of cylindrical crystal, the differential phase shift of light wave is recovered by means of two spatial modes (LP01 mode and even LP11 mode) transmitted in the optical fiber and interrogated by low-coherence interferometer. Except for capacitor divider, quartz crystal, it is a whole optical fiber scheme, no separated optical devices, such as: collimators, polarizer and wave plates; Thus the cost of sensor is reduced, accuracy is advanced, and it is convenient for production in scale. Finally, the paper offers a design scheme for the voltage transformers' system and insulator, studies the performance of voltage transformer and provides a theory gist for the application of high voltage transformer.