Abstract: Longitudinal-torsional ultrasonic vibration system is becoming more and more popular because it can improve machining efficiency and accuracy and increase surface strength. However, the torsional vibration component of the existing longitudinal-torsional vibration system is small and the conversion efficiency is low. Based on this, the composite modal ultrasonic vibration system with periodic structure is proposed in this paper, and its vibration modes, resonant frequency and other characteristics are analyzed and verified by numerical simulation. The results show that the two-dimensional periodic structure of comb sector holes and skewed slots can increase the longitudinal and torsional amplitudes, displacement curl, shear stress and normal stress, and improve the longitudinal-torsional conversion ability.