Abstract: In ultrasonic welding of polymers, the heating process can be considered as the process that the energy of sound field converts into the internal energy of polymers. According to the theory of second quantization, the energy flow of ultraosnic is equivalent to the phonons flow of a given flux. Phonons can only be absorbed as a complete unit by atoms. Atoms have separate energy levels and will absorb phonons in the transition from a lower level to a higher level. The total energy of these absorbed phonons is equal to the difference in energy of these two levels. According to this theory, in the process of one-vibrator welding, only the atomic vibrators with frequencies of an integer multiple of ultrasonic frequency can effectively absorb phonons. Whereas, in double-vibrator welding, besides the vibrators with the frequencies of integer multiples of these two ultrasonic frequencies, phonons can also be absorbed by the vibrators with the frequencies of the sum of integer multiples of the two ultrasonic freqiencies. Therefore, the absorption capacity of phonons in double-vibrator ultrasonic welding is superior to that of single-vibrator welding, that is, the utilization rate of ultrasonic field energy in double-vibrator welding is much higher. The results of the corresponding experiments indicate that the double-vibrator system spends only 3/8 of the time of the single-vibrator system with half of the power in completing the same welding; and in the same welding time, the double-vibrator system with 12/13 of the power of single-vibrator system can accomplish the same welding.