Abstract: An ultrasonic-assisted forced infiltration method using plastic welding equipment was proposed to realize the efficient preparation of polymer-based thermal conductive composites. The carbon nanopapers (CNPs) consisted by pure carbon nanotubes (CNTs) were prepared using the method of vacuum filtration. The CNPs were placed into uncured polydimethylsiloxane (PDMS) viscous resin. The plastic welding equipment with high frequency and high power ultrasonic source was applied for the ultrasonic treatment, and the liquid PDMS would gradually infiltrated into CNPs depending on the energy transfer effect of ultrasonic wave. The CNPs/PDMS composites with high flexibility and excellent thermal conductivity could be obtained after CNPs/PDMS being cured at 100℃ for 30 min. The experimental results show that under the same amplitude condition, the thermal conductivity of CNPs/PDMS composites would increase first and then decrease with the increase of ultrasonic duration. The thermal conductivity reaches the peak value of 5.781 W/(m·K) when the ultrasonic duration is 1.9 s. The applicability of ultrasonic-assisted forced infiltration method for the preparation of flexible thermal conductive composites is verified in this research work.