In ultrasonic cavitation, iridescent rings always occur around erosion pits on steel surface. These colorful halos can reflect the experienced temperature of the steel surface, but the reason for their formation is controversial. In this study, the development of an erosion pit and the iridescent ring around it on stainless steel (1Cr18Ni9Ti) surface was numerically investigated based on the energy transformation theory. The results revealed that the experienced temperature of ring areas with the shape of three-dimensional hemisphere could reach as high as 1685 K, and the position of material's highest temperature was exactly at the position of stress concentration.