A combined grinding wheel for ultrasonic machining and the determine method thereof. The combined grinding wheel for ultrasonic machining comprises: an outer grinding ring, an upper end thereof having a center taper hole and a plurality of outer grinding ring water holes located in a circumferential outer side of the center taper hole; a connecting taper shank, the upper end thereof having a taper shank extending into an outer grinding ring cavity from the center taper hole and matching the tapered surface of the center taper hole; a connecting flange, located outside the shank portion and fixed on the inner wall of the upper end of the outer grinding ring cavity, and having a center tapered through hole; and an inner vibration unit, having a tapered surface matching the tapered surface of the center tapered through hole and a center hole connected with the connecting taper shank by means of a screw.