The utility model relates to an open-close current transformer, comprising a coil, characterized in that an iron core of the transformer is composed of two circular arc iron cores which can paste up a circular ring, wherein the arc length of one small circular arc iron core is 25% to 40% of the arc length of the other large circular arc iron core; the large circular arc iron core is coiled with the coil; circular arc outer shells are separately arranged on the outside of the small circular arc iron core and the large circular arc iron core; fastening belts are disposed on the outside of the two circular arc outer shells which are closed into a circular ring; the iron core is the ultracrystalline alloy; and the fastening belt consists of a nylon belt and a belt button. The utility model has the advantages that the utility model can make the current transformer to be assembled and disassembled electrically and guarantee the normal operation of the power supply device.