SiC_f/SiC composites was processed by ultrasound-assisted grinding， and the effect of feed rate on the machining damage of the composites by MI process was studied. The results show that when the surface of SiC_f/SiC composite is processed by ultrasound-assisted grinding， many machining defects will come into being， such as the fiber debonding， fracture， breakage， matrix crack and shedding. And the interface between the fiber and matrix also has cracks. With the increase of feed speed， the surface damage of the composite material is aggravated， which leads to the decrease of proportional ultimate strength and maximum load. When the feed rate increases from 400 mm/min to 1 000 mm/min， the tensile strength and bending strength of SiC_f/SiC composites reduce by 4.7% and 20.6%， respectively.