The fiber optical current transformer (FOCT) has distinguishing advantages of small size,simple insulation,no magnetic saturation. It can be better compatible with modern digital control and protection systems,which has become an important development direction of current transformer. However,the reliability of FOCT in long-term operation is low at preasent,so its popularization and application in digital substations is seriously restricted. In this paper,the fault tree analysis of FOCT is comprehensively carried out from both the circuit and the optical path,and a fault mode analysis system covering the overall structure of FOCT is established. Firstly,the structure and operating principle of FOCT are analyzed,and the fault is divided according to its circuit composition. Secondly,the main fault modes occurring in the long-term operation of FOCT are extracted,and the listed fault modes are investigated one by one. Then,the reliability improvement measures are proposed from the aspects of scheme design,component selection and construction technology. Finally,the fault tree is used to analyze the cause of a light source fault combining with an actual case,and the reliability improvement measures are proposed to verify the effectiveness of the proposed method. The research can provide a theoretical basis for FOCT fault analysis and operation reliability.