The invention relates to the field of electrical equipment, in particular to an external current transformer. The external current transformer comprises a ring body used for enclosing a GIS barrel, the ring body is of a split splicing structure comprising two or more splicing sections, an optical sensing module used for measuring currents is packaged in the ring body, and the ring body is connected with a secondary conversion device used for measuring the currents according to signals of the optical sensing module. Due to the facts that the ring body form is adopted for the external current transformer, the ring body is of the split splicing structure comprising two or more splicing sections, and the optical sensing module is packaged in the ring body, when the external current transformer is used, the ring body can be combined on the GIS barrel of a sealed primary conductor of corresponding GIS equipment and does not directly intervene in the interior of the barrel any more, the integrality of the barrel can be kept, later-stage maintenance can be facilitated, and thus it can be seen that the problem that an existing current transformer used for a GIS is inconvenient to maintain is solved.