Using　optical　methods　for　measuring　electrical　currents　have　significant　advantages.　Optical　current　transducers　are　typically　done　by　means　of　the　Faraday　magneto-optic　effect.　All　fiber-optical　current　transducer　(AFOCT)　is　a　new　kind　of　optical　current　transducer,　with　many　merits　over　traditional　current　transducer.　It　is　usually　used　for　monitoring,　protection　and　measurement　in　high-voltage　power　grid.　This　thesis　presents　basic　theory　of　all　fiber-optical　current　transducer.　And　three　main　types　of　it　and　their　principles　are　introduced.　Linear　birefringence　in　optical　fiber　is　a　fundamental　reason　influencing　the　performance　and　sensitivity　of　all　fiber-optical　current　transducer,　so　it　severely　limits　practical　application　of　all　fiber-optical　current　transducer.　The　causes　of　linear　birefringence　in　optical　fiber　are　discussed　and　some　methods　are　devised　to　solve　this　problem.　They　are　including　using　some　special　kinds　of　optical　fiber,　such　as　low　linear　birefringence　optical　fiber,　high　circular　birefringence　optical　fiber,　spun　optical　fiber,　twisted　optical　fiber　or　annealed　optical　fiber.　Light　source　and　other　optical　devices　also　introduce　measurement　errors.　It　is　important　to　choose　appropriate　optical　devices.　This　paper　discusses　their　effect　upon　all　fiber-optical　current　transducer　and　suggests　several　solutions　to　reduce　measurement　errors.　And　some　environmental　factors,　such　as　temperature　fluctuation　and　vibration　also　affect　all　fiber-optical　current　transducer　significantly.　So　the　temperature　compensation　and　vibration-proof　measures　are　also　essential.