An ultrasonic monitoring probe for internal service stress of a marine structural component. The probe includes a detection wedge provided with two symmetrically arranged inclined surfaces at its top, two connecting channels vertical to the two inclined surfaces and penetrating through the detection wedge and provided with threaded holes close to the inclined surfaces and water storage cavities far away from the inclined surfaces, two ultrasonic transducers mounted in the threaded holes of the two connecting channels and configured for generating and receiving ultrasonic waves; two bottom rings located at a bottom of the detection wedge and arranged relative to the water storage cavities and configured for attachment to a surface of a detected component, a magnet disposed in a magnet placement hole arranged at a central position between the two connecting passages, and a monitoring device electrically connected with the two ultrasonic transducers.