The invention relates to the technical field of electric power, and discloses a trimming device for solving a problem of an irregular shape of an insulating layer of a square tube. The trimming device comprises a shell, wherein an air ring is fixedly connected to the interior of the shell. Through the combined action of the air ring, a detection mechanism, an insulating tube, a metal slip sheet, an ejector rod, an electromagnet, a brake rod, a first rotary disc and a ratchet wheel, heating operation is automatically triggered by utilizing a bulge on the surface of a cable, so that the bulge of the insulating layer is broken by heating, the trimming operation is facilitated, the trimming operation is started, the linkage between structures is improved, the operation is convenient and fast, and the automation degree is high; and through the combined action of a second rotary disc, a sliding plate, a third rotary disc, a pressing tool and a ring plate, the four faces of the cable are extruded automatically, the other faces of the cable are supported while the insulating layer face where the bulge exists is flattened, the situation that the cable is distorted due to non-uniformity is effectively avoided, and the trimming quality is guaranteed.