Ultrasonic precise bonding is an emerging technology in the application of polymer micro-assembly. In the ultrasonic bonding process, the propagation of ultrasound varies with the change of the interfacial polymer physical state. So the ultrasonic guided wave is an effective parameter to in-situ monitor the fusion degree. The ultrasonic guided wave in the ultrasonic bonding process is studied by vibration analysis and online visual monitoring in this paper. The time-frequency characteristics in the ultrasonic guided wave in the bonding process are mainly analyzed by Fast Fourier Transform spectrum analysis, Wavelet Packet Decomposition, and envelope spectrum methods. The polymer interfacial fusion is monitored by the high-speed HD camera in the ultrasonic bonding process. The time-frequency characteristics in the ultrasonic guided wave and the fusion behavior of the thermal melt interface are analyzed and correlated. Results indicate that the change of the interfacial thermal melt state is related to the time-frequency characteristics of the ultrasonic guided wave. The fusion of the melting zone, the rotation of the micro-device, the generation or disappearance of local air bubbles all lead to the changing of the harmonic frequency and intensity in the ultrasonic bonding process.