PDC bits have the advantages of high rate of penetrate （ROP）， long service life and flexible design when drilling in soft to medium-hard formation. The demand of PDC bits in drilling field is increasing yearly. As a new type of rock breaking technology， ultrasonic drilling has attracted wide attention due to its strong penetration and low drilling cost. However， the understanding of the formation mechanism of rock cuttings at micro level and the change of cutting force in the cutting process is not enough， which makes the wide application of this technology limited. In order to deeply understand the cutting mechanism of ultrasonic drilling， based on the linear Drucker-Prager model and ABAQUS software， a two-dimensional finite element model for ultrasonic vibration assisted rock cutting is established， and the formation mechanism of rock cuttings at micro level of ultrasonic vibration cutting is analyzed and compared with the conventional single tooth cutting process. In addition， the variation law of rock breaking specific work and cutting force at different frequencies is analyzed. The results show that， in the process of ultrasonic vibration assisted cutting， the cutting force decreases to zero more obviously than conventional cutting； when the excitation frequency is close to the natural frequency of rock， ultrasonic vibration cutting has smaller cutting force than conventional cutting， which is conducive to improve the stress state of the tool and reduce the wear degree of the tool； ultrasonic vibration cutting is easier to produce large blocks than conventional cutting when the frequency increases from 20kHz to 40kHz； the rock breaking specific work and average cutting force decrease first and then increase， indicating that there is an optimal frequency at which the specific work of rock breaking is minimum while drilling efficiency is highest.