Face gears are the key basic components for high-end equipment such as main reducing gear of new helicopter. However, the existing face gear processing methods are difficult to ensure the profile accuracy and surface quality at the same time, combined the advantages of 5-axis ultra-precision single-point diamond turning technology in complex surfaces and the characteristics that ultrasonic vibration can reduce the chemical wear between gear steel and diamond tools, a new method of machining face gear is proposed in this study, which is 5-axis ultra-precision single-point diamond turning with ultrasonic vibration assisted. Firstly, the cutter contact (CC) points solution model of face gear surface is established, by projecting the Archimedean spiral discrete points onto the tooth surface of the face gear, the coordinates of the CC points on the left tooth surfaces and right tooth surfaces of the face gear are obtained. Next, combined with the motion chain of 5-axis lathe, the interference mechanism of single point diamond turning face gear is analyzed, meanwhile, the limit position of the face gear turning was constructed and the allowable swing range of the tool shaft was further determined. Furthermore, the influence of the cutter parameters and the cutter holder parameters on the feasible region of tool axis is studied. Finally, based on the kinematic analysis method of multi axis machining, the subordination of each motion axis of the 5-axis turning is analyzed, and the kinematics model of 5-axis single-point diamond turning is established.