Abstract: To improve the lower mechanical property in E40 steel joints obtained by conventional underwater wet welding, this article presents a novel underwater wet welding method through applying ultrasonic vibration to the work piece in front of the welding pool. The effect of ultrasonic vibration on the weld formation, microstructure and mechanical properties of underwater wet welded joint are investigated experimentally. The experimental results are compared with those by conventional underwater wet welding under the same welding conditions. The results show that ultrasonic vibration could reduce the asymmetry of weld cross-section, and enlarge the weld depth-to-width ratio. In addition, the grain refinement induced by ultrasonic vibration is usually related to the large instantaneous pressure and temperature fluctuations generated in the weld pool, which can increase the probability of nucleation and dendrite fragmentation, leading to grain refining in the weld metal during solidification, and lower tendency of column crystallization. Compared with conventional underwater wet welding, the tensile strength obtained by ultrasonic vibration assisted underwater wet welding is improved to some extent due to the introduced ultrasonic vibration, about 92.4% of the base metal. In addition, the microhardness in the weld zone is increased, while that in the heat affected zone has no obvious change. Therefore, it is supposed to be a good way to improve weld properties for underwater wet welding.