A new method for design ultrasonic horn was presented to solve the existing problem, which is based on the theory of onedimensional longitudinal vibration. The frequency equation and formula for calculating amplitude radio of sinusoidal type ultrasonic horn were deduced, and it was showed that ultrasonic horns with variable shapes and amplitude radios could be required through altering the initial phase of sinusoidal generatrix when area coefficient is invariable. The compound sinusoidal horn with cylindrical rod at the narrow end was designed. The theoretical derivation, and dynamical characteristics based on ANSYS software, indicate that while choose the reasonable initial phase and length of sine section, the compound horn has more advantage than stepped horn in amplitude radio and form factor.