The invention provides a cutting backing structure of an underwater diamond wire saw, comprising two support seats and a backing board, wherein each support seat is provided with a backing board supporting part and reinforced ribs, both ends of the backing board are respectively arranged in the backing board supporting parts of the two support seats, and the board body of the backing board is provided with a guiding part. When the wire saw works, the backing board enters and stays in a notch with a bead rope, and the notch dose not close when a platform to be cut falls down. The cutting backing structure of the underwater diamond wire saw effectively avoids abnormal running of the wire saw caused by that the bead rope is locked because of closing of the notch, thereby continuously implementing the cutting process, and increasing working efficiency and construction quality.