The utility model discloses a desilting structure inside a large ship water ballast tank, which comprises a ship bottom board, a plurality of rib boards, a plurality of longitudinal beams and a plurality of drain holes. The rib boards are arranged in parallel on the ship bottom board along the direction of the width of a ship equidistantly, the longitudinal beams are arranged between adjacent rib boards in parallel along the direction of the length of the ship equidistantly, the equidistant drain holes are arranged on the longitudinal beams, and the drain holes on each adjacent longitudinal beams are in displacement arrangement, in other words, the drain holes on each adjacent longitudinal beams are not on the same cross section. The desilting structure effectively removes accumulated silts inside the ballast tank, relieves labor intensity of shipman removing the silts, and improves economical benefits of running the ship.