The invention discloses an underwater detecting device. The device comprises an underwater moving device, a lifting device, a detecting device body and an information transmitting device, wherein thelifting device is arranged above the underwater moving device; the detecting device body is arranged under the tail of the underwater moving device; the information transmitting device is arranged onthe lifting device. According to the technical scheme, the buoyancy of the underwater detecting device under water is controlled by the volume of a closed cavity formed by a piston and an inflation cylinder, the volume of the cavity is changed through the deformation of an elastic inner membrane, pressure difference is used for sucking or discharging objects in the cavity through a water inlet check valve and a water outlet check valve, and the device moves by utilizing the reverse thrust generated by the discharged water.