The model of ultrasonic propagation at a solid-solid contact interface was established. Higher-order harmonic wave generation and waveform distortion take place when the ultrasonic wave propagation at the solid-solid contact interface. Aluminum wire bond experiments were performed on a laboratory test bench. The relation of bond strength, nonlinear coefficient and contact interface pressure was studied. The experiment results show that when contact interface pressure is less than 6 kPa, higher-order harmonic wave component and the nonlinear coefficient decrease and bond strength increases with contact interface pressure increasing, when contact interface pressure is in range of 6 kPa to 10 kPa, higher harmonic wave and the nonlinear coefficient is the least and bond strength is the highest, however, when contact interface pressure is more than 10 kPa, the nonlinear coefficient increases and bond strength decreases with pressure increasing. The nonlinear coefficient of ultrasonic is a method of forecast bond strength.