The invention provides an underwater releaser, consisting of an electromotion part, a trigger mechanism, an energy storage mechanism, a speed reduction mechanism, a bidirectional overflow mechanism, a dynamic sealing transmission mechanism and a releasing arm, wherein the electromotion part is positioned inside a pressure-resistant cabin; a lower bottom plate of the pressure-resistant cabin is connected with an isolating cabin so as to jointly form a seawater isolating cabin segment in a circle; the seawater isolating cabin segment is provided with the bidirectional overflow mechanism; and the trigger mechanism, the energy storage mechanism and the speed reduction mechanism are all arranged inside the seawater isolating cabin segment. The underwater releaser adopts the design of combining two blocks of U-shaped magnetic steel, ensuring that the maximum trigger power is close to the scalar sum of the gravitational force and repulsive force between the U-shaped magnetic steel; by using a plane volute spring to store energy and using a gear set to reduce speed, the underwater releaser can provide great releasing power; a seawater isolating cabin is built through the bidirectional overflow design, which avoids seawater impact and the adverse effect of marine organisms; through a rotary unhooking mechanism, the releaser can have greater bearing capacity. The underwater releaser is not only better in watertight reliability, but also can work underwater for a long period of time without replacing components.