In this paper, the stepped horn node position and its deviation with the theoretical value were analyzed. And the main reason which leads to the difference between the real position and its theoretical position was also discussed. Because of the existence of Poisson’s ratio, the longitudinal ultrasonic vibration will induce horizontal vibration, and this will affect the velocity of ultrasonic vibration, finally lead to the deviation of the node position. The optimal node position was analyzed by finite element method. The numerical analysis shows that offsetting node position to the small end of horn can reduce longitudinal vibration of the support plate.