In order to increase the bonding strength, decrease the nonlinear vibra tion of ultrasonic transducer system and enhance the stability of chip bonding, this paper described the ultrasonic propagation at the contact interface in a th ermosonic bonding system and presented a micromechanical model of ult rasonic propagation. When the static stress is small, the total stress is smalle r th an zero. The contact interface of two materials will detach, which makes the wav efor m of output ultrasonic imperfect, and the displacement of material particle vibr atio n is small. When the static stress increases, the materials enter elastic def ormation phase and the output ultrasonic waveform is identical to the input ultr asonic waveform with the maximum displacement of the material particle vibration . When the static stress is large enough, the materials enter plastic phase. Residual stress and residual displacement exist in interior material because of loading and unloading of dynamic stress, and hardening of material comes into be ing, which leads to the distortion of output ultrasonic waveform and the decreas e of material particle vibration displacement. An experiment is conducted and the r esults are testified by the model.