The invention relates to a tensioning device of an underwater diamond wire saw, which comprises a tensioning wheel, a tensioning wheel axle seat, a tensioning force detection mechanism, an adjustment handle, a screw rod and a base, wherein the inner side surface of the base is in slide fit with the tensioning wheel axle seat, while the outer side surface is connected together with a frame of the diamond wire saw; the screw rod is positioned inside the tensioning wheel axle seat, and one end of the screw rod passes through the tensioning wheel axle seat to be connected and fixed with the adjustment handle; one end of the tensioning wheel axle seat is provided with an axle seat hole for the tensioning wheel axle seat to pass through so as to be positioned; and the tensioning force detection mechanism is sleeved on the screw rod and is positioned at one end of the tensioning wheel axle seat away from the axle seat hole. The tensioning device can ensure the initial tensioning force of a wire saw string bead wire, stable tensioning force during machining and convenient assembly and disassembly of the string bead wire; moreover, the tensioning device carries out realtime monitoring and controls the radial feeding speed of the string bead wire through a tension sensor, thereby ensuring safe, smooth, efficient and accurate cutting operation; therefore, the tensioning device can meet the requirements of large-diameter or multi-material filled pipes for carrying out cutting operation.