The invention discloses a reinforcing device used for an offshore lassen steel sheet pile and a construction method thereof. The device comprises U steel, a supporting rod, a bolt and a fixed plate. The side portion of the U steel is welded to the outer wall of a steel sheet pile, two wings of the U steel are vertically arranged and are perpendicular to the steel sheet pile, an opening between thetwo wings is formed in a manner of being backing on to the steel sheet pile, a first thread hole and a second thread hole are formed in the two wings horizontally, and the first thread hole and the second thread hole are used for allowing bolts to horizontally penetrate through. The supporting rod is inclined, the upper end of the supporting rod is horizontally provided with a third thread hole,the lower end of the supporting rod is used for stretching into the sea water, and a bolt is used for fixing the upper end of the supporting rod between the two wings of the U steel. The fixed plate is made of steel, located below the supporting rod and in an L shape, and the vertical segment of the fixed plate is perpendicular to the horizontal segment, and is inserted into underwater silt. The supporting rod is arranged for supporting the steel sheet pile, the steel sheet pile is not prone to toppling, a fixed plate is arranged for supporting the supporting rod, and after bearing the pressure of the supporting rod, the fixed plate compacts the sludge.