The formation of a ring area around the cavitation erosion pit on carbon steel is due to the cavitation erosion–corrosion. For the ultrasonic irradiation in water, both the ultrasonic cavitation and the acoustic streaming are generated. Due to the oscillation of the horn, the acoustic streaming flows from the tip of the horn to the bulk water. So far, the acoustic streaming has not been considered in previous studies on the ultrasonic cavitation erosion–corrosion. This study first reveals that the acoustic streaming noticeably affects the evolution of the nascent ring area. The mean velocity of acoustic streaming is inversely proportional to the gap (H) between the tip of the horn and the surface of the specimen. When H is 65 mm and 40 mm, the corresponding ring area is mainly composed of vertical sandwich sheets. In contrast, when H is 17 mm, the corresponding ring area is mainly composed of horizontal thin sheets. This study provides a complete picture to understand the ultrasonic cavitation erosion–corrosion on carbon steel.