Cavitation erosion is accomplished by a magnetostrictive transducer at different experiment input powers using column steel samples. The distribution characteristics of bubbles on the emitting surfaces are observed at different times. The photo records show that ultrasonic bubbles distributed on the emitting surface will converge and form some interesting patterns of branches, which varies with the increase of time and different powers. Such phenomena are explained by the theory of viscous fingers effect and the mode of diffusion limited aggregation. The cavitation erosion appearances of the samples are measured by a three-dimensional (3D) surface measuring system. The results show that the erosion degree and characteristic on the sample emitting surface are dominated by the input powers. It is concluded that the cavitation erosion appearance is decided by the distribution structure of bubbles and the degree of cavitation erosion is influenced by the density of bubble group and the thickness of the bubble layer.