Ultrasonic vibration rock fragmentation technology is a new method to solve hard rock drilling problems. Its technical feasibility has been verified by a large number of scholars at home and abroad. However， there is still insufficient understanding of the hard rock fragmentation mechanism under ultrasonic vibration. There is an inevitable connection between the surface radial response displacement of rock and its internal damage state under ultrasonic vibration. In this paper， by monitoring the radial response displacement of rock surface at different depths during ultrasonic vibration， the relationship between the spatial-temporal evolution of surface radial response displacement of rock at different depths and the development of its internal damage is analyzed by using the theory of stress wave propagation. The results show that the damage of rock under ultrasonic vibration is mainly caused by Hertz cone-shaped circular crack caused by high-frequency impact of the vibrating head and fatigue damage caused by the alternating stress of ultrasonic vibration. The propagation depth of Hertz cone-shaped circular crack is 10mm， and the fatigue damage crack mainly occurs at the depth of 10 to 20mm. There is obvious indication of radial response before the local macro fracture of rock under ultrasonic vibration. The radial response displacement of rock surface can be used as the failure criterion under ultrasonic vibration. The research in this paper is of great significance for enriching the mechanism of hard rock breaking under ultrasonic vibration