The optical and electrical components of fiber optical current transformer (FOCT) are studied. The principle and structure of the system are described. By means of FMECA and FTA the reliability analysis of the system is made. Through the fault cases in practical application, the critical component failure modes of the system are determined. The fault tree of the optical unit is constructed. According to the risk priority number of failures the design weak points are determined and the improve measures are put forward. In this way the risks of critical components failure can be reduced. The fault cases of FOCT are analyzed and the validity of the system reliability analysis method is verified.