Fusion precision is an important factor to be considered for ultrasonic bonding in thermoplastic MEMS. Vibration transmission in ultrasonic bonding process through polymer workpiece was related to the interfacial fusion, which could be used for in situ monitoring. In this article, experimental platform was established to study the vibration transmission. Experiments were carried out to measure the vibration transmitted from horn onto the high‐frequency dynamic force sensor installed in the anvil. According to the peak‐to‐peak value of dynamic force, the whole bonding process could be divided into rising and falling phases. Interfacial fusion corresponding to ultrasonic transmission was also studied. From experimental results, when the peak‐to‐peak value change rate rose to 130%, some area on the surface starts to melt. When it fell down back to 60%, glass transition occurred with the melt spread gradually over the whole interface. When it fell down to 30%, defects of overflow and bubble began to generate as polymer got into viscous flow state. When the change rate reached 10%, the bubbles burst into smaller ones like floccules as the fluidity of polymer was enhanced further under continuous ultrasonic load. The experimental results gave a deep understanding in the formation of interfacial fusion and the generation of defects.