The invention relates to an electroslag casting device additionally provided with electromagnetic stirring and a method thereof. The electroslag casting device belongs to the technical field of special steel electroslag metallurgy and comprises a crystallizer, a crystallizer water jacket and an electromagnetic stirrer; an annular magnet is sleeved on the outer wall of the crystallizer and positioned at the middle-upper part of the crystallizer, a loop is wound on an iron core of the annular magnet, an electromagnetic stirrer water jacket is arranged at the periphery of the annular electromagnet, and the electromagnetic stirrer water jacket is communicated with the crystallizer water jacket; a melting consumable electrode forms molten steel drops which then enter a slag layer; the molten steel drops fall down in a spiral line way in the slag layer under the action of the rotary electromagnetic field and then enter a steel ingot liquid tank; the steel ingot liquid tank raises continuously and also enters into an effective magnetic field action region of the electromagnetic stirrer; and meanwhile, electric current in the molten steel is acted with the rotary magnetic field to produce the electromagnetic force which then drives the molten steel to rotate. The invention has simple equipment, and has the obvious functions of improving the surface quality, the subsurface quality and the solidifying tissue of a casting blank and the isometric crystal ratio, and lightening the internal defects, such as the center segregation, the center looseness, and the like.