With its advantages of freedom from saturation and large dynamic range，the fiber-optic current transformer（FOCT）has definite applications in smart substations and DC transmission projects.Based on the principle and structure of FOCT，its influence on the AC and DC transient response characteristics and protective application is dealt with.The influence of such factors as the Verdet constant，particle nature of light，ambient temperature，optical components aging on the transformer accuracy and protection is studied.Also analyzed is the problem of protection reliability brought about by the characteristics of digital sampling，fiber installation and polarity.Countermeasures are proposed as the measures to improve the reliability of FOCT in application.