As a new welding method, ultrasound has been successfully introduced into the pool during ultrasonic wave-assisted arc welding process. However, the interaction mechanism between the ultrasound and the arc plasma is not clear, thus preflenting the new technique from engineering applications. In this paper, the characteristic of arc regulation by external ultrasonic field is investigated based on the experimental data and the corresponding theory. In order to figure out the characteristics of arc, the arc images obtained by high-speed camera are processed. Compared with the conventional welding arc, ultrasonic wave-assisted arc is more contracted and becomes brighter, the high-temperature region in an arc column greatly expands, and there are internal particle agglomerations shaking up and down at a constant frequency. The arc shape varies with ultrasound excitation current and the height of ultrasonic radiator. In the vicinity of the resonance point, the straight-degree of the arc is the strongest and the ripple frequency is also the largest. Results show that the purpose of using external ultrasound field to regulate the thermal plasma has basically achieved. Analyzing the acoustic pressure wave equation for the neutral component shows that the spatial distribution of acoustic wave can be generated in the arc and its intensity is proportional to the local amplitude of acoustic waves. Acoustic pressure field can be calculated based on the dependence of the electron temperature and density on time and space. In addition to the action of acoustic field within the arc, the arc plasma is also controlled by the acoustic field structure. A two-cylinder model incorporating boundary element method is developed, establishing a relationship between the binding capability and the geometric parameters of an ultrasonic radiator with reflerence to wavelength. This model is successful in predicting resonant modes of the acoustic field and explaining the influences of the ultrasonic radiator height on welding arc. Variation of arc shape is the result of the combined effect of axial and radial acoustic radiation forces on particles (electron, ion and neutral). The thermal efficiency will be significantly enhanced since the particle density increases in the ultrasonic wave-assisted arc. The acoustic propagation in the arc is the interacting process between acoustic and thermal plasmas. The mechanism of ultrasound acting on the arc can be reasonably explained in this study. And the results may provide a reflerence for plasma engineering applications. However, it also needs further reflearch on the impact of an arc on the acoustic field.