The utility model discloses a prepressing tool for sealing secondary coils of current transformers. The prepressing tool comprises a CT (current transformer) shield, a U-shaped draw bar and a plurality of layers of secondary coils; a flange is arranged at the bottom of the CT shield, and the secondary coils are sleeved on the CT shield; the flange at the bottom of the CT shield is placed on the bottom surface of the U-shaped draw bar; and a pressure plate is arranged on the secondary coil on the top layer, a circular hole is formed in the center of the pressure plate, the CT shield can penetrate the circular hole, strip-shaped holes are arranged on side surfaces of the pressure plate, and the U-shaped draw bar is inserted into the strip-shaped holes to be fixedly connected with the pressure plate. The prepressing tool has the advantages that the prepressing tool is provided with the U-shaped draw bar and the pressure plate, planes which are positioned among the secondary coils and the CT shield and are required to be sealed are compressed under a matching effect of rubber strips, polyurethane pouring materials are directly poured on the planes, and the tool can be detached after the polyurethane pouring materials are solidified; after a sealing process is optimized, the prepressing tool is simple in structure, and workload is reduced; the solidifying time for silicon rubber plane sealants is omitted, so that the complete production cycle is shortened; and sealing gaps of the sealed planes can be adjusted by means of increasing the pre-pressure of the tool if the polyurethane pouring materials leak during pouring.