Abstract: The heat induced by friction and viscoelastic effect in the ultrasonic welding thermoplastics was calculated via multi-physics coupling model. Based on the calculation, influences of ultrasonic amplitude on the temperature field of weld zone were studied. Moreover, the existance of critical amplitude, below which the thermoplastic would not flow, was discovered during ultrasonic welding. Temperature-measurement test was carried out to verify the simulation. The simulation results agreed well with experiment results. And the temperature field under critical amplitude was discussed. The study of critical temperature field in this paper will contribute to solving the problem of temperature controlling during ultrasonic welding and to laying the foundation for non-melting joining of thermoplastics using ultrasonic vibration.