The invention discloses a plate beam transverse connecting device and a construction method thereof. The plate beam transverse connecting device comprise a first connecting piece and a second connecting piece, wherein the first connecting piece and the second connecting piece are respectively arranged in adjacent plate beams and have the same structure; the first connecting piece and the second connecting piece are each of an integral U-shaped structure, any vertical section of the transverse parts and the vertical parts of the first connecting piece and the second connecting piece is in an Lshape, the first connecting piece and the second connecting piece are in butt joint along with splicing of the adjacent plate beams to form a rectangular limiting groove, steel plate connecting piecesare flatly laid at the bottoms of the rectangular limiting groove, and reinforcing steel plates are arranged on the steel plate connecting pieces at equal intervals in the height directions of the first connecting piece and the second connecting piece, wherein the space between the steel plate connecting pieces and the reinforcing steel plates is filled with hinge joint concrete, and the space between every two adjacent reinforcing steel plates is filled with hinge joint concrete. The plate beam transverse connecting device is simple in structure and low in cost, can improve overall stress performance in a multi-plate-beam structure, ensures structure safety, has small influences on an original plate beam and can be applied to new construction and reinforcement engineering.