The work aims to improve surface quality of welded joints. Ultrasonic impact treatment (UIT) was applied to weld toes of 16 MnR cross joint, and comparison test and comparative analysis were applied to surface quality of weld toes as-welded and treated under different UIT parameters. After UIT, transition radius at weld toes of welded joint increased, and transition from base metal to weld became smooth. Surface microstructure of weld toes was significantly refined, and refined depth and refining degree varied with UIT parameters. Tensile stress state of the weld toe as-welded transformed compressive stress state, surface microhardness increased greatly after UIT. The compressive stress was the maximum (269.8 MPa) and surface microhardness was also the maximum (340.5HV) under the effect of 1.5 A/20 min UIT. Meanwhile, surface roughness tended to increase after UIT, and improper UIT would lead to different kinds of defects on the surface of welded joint, thus affecting overall surface quality of welded joint. Surface quality of joint depends on not only impact time and impact current, but also location among impact points during single-channel impact and impact channels during multi-channel reciprocating impact as well as tightness of connection. Not all surface quality indicators are positively related to or linearly dependent on ultrasonic impact parameters.