The invention belongs to the technical field of metal additive online detection, and especially relates to a water immersion ultrasonic online detection system and method in a metal additive manufacturing process. The system comprises a printing system, a water immersion ultrasonic signal excitation and detection system, a motion control system and a water circulation system. Through the integrated integration of the water immersion type ultrasonic online detection system and additive manufacturing equipment, plane scanning of the lower surface of a substrate is realized by a water immersion type ultrasonic probe, and online nondestructive detection of additive manufacturing organizations and defects is realized on the basis of a reflection echo of a printing layer. According to the waterimmersion ultrasonic online detection system provided by the invention, mutual independent operation of water immersion ultrasonic scanning detection and the additive manufacturing processes is realized, and mutual interference is avoided; adaptive process regulation and defect online repair are realized, and the quality of additive manufacturing parts is effectively improved; and the stability ofthe water immersion ultrasonic detection system is high, the influence of factors such as vibration, electromagnetic interferences and rough surface and the like in the additive manufacturing processon the accuracy of a detection result can be effectively inhibited, and the detection precision is high.