The material removal mechanism of the high-volume fraction SiCp/Al composites in ultrasonic assisted machining was elucidated by ultrasonic assisted scratch experiment. The influence of scratching parameters on the ultrasonic amplitude, scratching force and friction coefficient were investigated by the experiment. The morphology of the scratch grooves was observed with the scanning electron microscope and the laser confocal microscopy. Further, the characteristics of ultrasonic assisted machining was analyzed by ultrasonic assisted scratch experimental date. The experimental results show that the current value decreases gradually with the increase of scratching depth from 0.01 mm to 0.05 mm, the change amount of current value raises from 12 mA to 25 mA, the ultrasonic amplitude decreases gradually, the axial impact of diamond indenter weakens; the extrusion cutting effect of tangential direction is enhanced with the increase of scratching depth and scratching speed, the scratching force and friction coefficient are improved. During the process of scratch, the SiC particles are broken into small particles, sheared fracture and some SiC particles are pulled out. The aluminum matrix appears obvious plastic flow and coating phenomenon, forming the accumulation outside the scratch grooves. Thus, when the cutting depth and feed speed are small, the material removal is mainly completed by the action of axial high-frequency vibration impact, the quality of the machining surface is higher; when the cutting depth and feed speed gradually increase , the material removal is completed by the combined action of axial impact crushing and tangential extrusion cutting, the quality of machining surface is gradually worse.