<p>Cf/SiC ceramic-matrix carbon fiber reinforced composites are widely used in the aerospace industry due to their excellent thermo-mechanical properties. However, the peculiar fiber-reinforced composite structure makes its failure mode different from that of ceramic materials, leading to a deviation between the existing cutting force model and the actual production. In this paper, considering the microstructure of C_f/SiC composites, the transient grinding effect of abrasive grains on fibers with different laying directions during ultrasonic-assisted grinding was analyzed, and three typical failure modes of fiber-matrix composite rods were proposed, including normal shear, axial crushing, and radial bending. Then, the energy dissipation process under each material failure mode was analyzed, and the composition and calculation method of microscopic specific energy were proposed. Finally, the grinding force model of ultrasonic-assisted grinding of C_f/SiC was established. The experimental results show that the average prediction deviation of the model is less than 11.14%. In particular, ultrasonic vibration has a significant effect on the instantaneous specific energy, which can reduce the grinding force by more than 20% compared with conventional grinding. The results of the specific energy analysis further show that the prediction accuracy of the model increases with an increase in instantaneous specific energy. However, when the specific energy is lower than 900&#xa0;J/cm³, the material removal mode changes from micro-brittle removal to macro-brittle removal, which increases the prediction error of the model.</p>