The invention relates to an underwater spanner used in underwater operation. It is formed by underwater motor, soft axle, the muffle with clamp device, and spanner support. Wherein, the underwater motor is mounted above the spanner support which is surrounded with thruster and handle the soft axle with high strength and flexibility is mounted inside the spanner support to mesh the axle of underwater motor, to form a flexible part to match the different angles of screw hole the changeable muffle that can clamp bolt or detach the bolt is mounted at the bottom of soft axle to fix the bolt and muffle as one whole that moving with soft axle. The invention can automatically position the spanner and pressurize the bolt, while it can prevent spanner rotating via the reverse torque of water. The invention has high flexibility, high strength, high automatic degrees and multi functions.