Combined cofferdam structure capable of being constructed on water

The utility model relates to a combined cofferdam structure capable of being constructed on water. The combined cofferdam structure comprises a lower structure, a middle structure and an upper structure which are sequentially arranged from bottom to top. The lower structure comprises two rows of cast-in-place piles, a plain concrete diaphragm wall and jet grouting piles, the cast-in-place piles are vertically arranged in parallel, the plain concrete diaphragm wall is arranged between the two rows of cast-in-place piles, and the jet grouting piles are arranged on the inner sides of the cast-in-place piles; the middle structure comprises a riprap foundation bed, a gabion retaining wall and a filling cofferdam, the gabion retaining wall is arranged on the upper portion of the riprap foundation bed, and the filling cofferdam is arranged on the right side of the gabion retaining wall; the upper structure is of a counterfort type structure and comprises a bottom plate, a vertical plate and rib plates, the bottom pl