As an important part of power ultrasonic vibration system, ultrasonic horn can amplify the displacement of mechanical vibration and concentrate energy on a smaller radiating surface, and the amplification coefficient of ultrasonic horn is a vital parameter in vibration system. The longitudinal vibration wave equation of conical ultrasonic horn is established according to differential element method and solved by the method of separation of variables and boundary conditions, to attain the mathematical expression between amplification coefficient and three variables called the ratio of the small end to the large end diameter, length and external excitation frequency. Two of the three variables are set successively as constants, and then amplification coefficient can be achieved according to the third variable sampling value by the mathematical expression. The sample calculating data of ultrasonic horn is applied to get fitting curve by Matlab, and relationships between amplification coefficient and three variables are analyzed, qualitatively. 