Abstract: Objective The aim is to study effect of ultrasonic vibration on microstructure and mechanical properties of 308 stainless steel arc surfacing layer. Methods 308 stainless steel is surfaced on the surface of Q235 carbon structural steel by metal gas shielded arc welding process, and the problem of coarse grain in the surfacing layer is solved by applying ultrasonic vibration to substrate during the surfacing process. Using optical microscopy (OM), electron backscattered diffraction (EBSD) technology and other test methods, effect of ultrasonic vibration on macroscopic morphology, grain morphology, size and orientation of surfacing layer is investigated. Effect of ultrasonic vibration on mechanical properties of surfacing layer is characterized by hardness experiments. Results The results show that application of ultrasonic vibration can change macroscopic morphology of weld pass, increase penetration, decrease weld width, and increase fusion ratio. Cavitation effect and acoustic flow effect produced by ultrasound in the molten metal can promote homogenization of solute elements, refinement of grains and transformation form columnar dendrites to equiaxed grains. EBSD results show that under the action of ultrasonic vibration, average size of grains decreases from 17.6 μm to 17.3 μm, content of γ-phase increases from 67.1% to 75.1%, and large-angle grain boundaries increase. Hardness of cladding layer increases from 40.3 HRC to 43.7 HRC. Conclusion The introduction of ultrasonic vibration into the arc surfacing process can improve microstructure and enhance mechanical properties of surfacing layer.