The invention discloses an underwater sealing device for an electromagnetic coupler. The underwater sealing device is composed of a device body, wherein the device body comprises a magnetic core sealing device, a circuit sealing device and a buckle device which are fixedly connected from left to right in sequence; and the magnetic core sealing device comprises an upper half magnetic core sealing device and a lower half magnetic core sealing device which are fixedly connected. The underwater sealing device can guarantee that the electromagnetic coupler is not influenced by environmental factors such as underwater pressure and corrosion, and is convenient to manufacture and install, so that tight lamination when the two half electromagnetic couplers are buckled on a transmission cable is guaranteed, the coupling coefficient is increased, and the underwater sealing device is applicable to sealing the electromagnetic coupler under different water depth environmental conditions.