The utility model relates to a construction machine for an in-water foundation in bridge engineering, in particular to an adjustable horizontal positioning device for a super-large steel hanging box. The adjustable horizontal positioning device is characterized in that sleeves (7) are internally arranged on a steel hanging box wall plate (1); an expansion push rod (3) is spliced in each sleeve (7) and connected with a steel protective sleeve wall plate (9) through an arc-shaped push support (8); a jack (2) is arranged on one side of the sleeve (7), and the other side thereof is connected with a sleeve stiffening rib (6); one end of the jack (2) is connected with the steel hanging box wall plate (1), and the other end thereof is pushed against a reaction plate (4); and the reaction plate (4) is vertically connected with a reaction plate stiffening rib (5), and fixedly connected to the tail end of the expansion push rod (3) and the arc-shaped push support (8); each group of mechanism units are horizontally provided with a pair of sleeves (7) in parallel; and a transversal connecting link (10) is arranged between two sleeves (7). The adjustable horizontal positioning device has the advantages of convenient accurate adjustment and better fixing effect.