Carbon nanotube (CNT) nanopaper (NP) has become a promising platform to fabricate light weight multifunctional polymer nanocomposites, because of superior properties of CNTs including low density, high strength and modulus, high electrical and thermal conductivities, etc. However, the low permeability of CNT NP (10-17 to 10-19 m2) makes it difficult to be impregnated by resin to fabricate nanocomposites via conventional approaches, such as resin transfer molding and hot melt compression molding. The resin infiltration time is usually more than one hour, and uniform resin impregnation is difficult to be achieved. Additionally, solvent is required to decrease the resin viscosity to facilitate the resin infiltration. All these make the process environmentally unfriendly and significantly limit the industrial applications of NP/polymer nanocomposites. Therefore, a novel ultrasonic assisted infiltration technology is developed to fabricate the NP/polymer nanocomposites with a uniform nanoparticle dispersion and high loadings (>20 wt%). The high frequency (20 kHz) ultrasonic vibration allows to effectively infiltrate various types of resins, including thermoplastic polyurethane (TPU), thermoset epoxy (EP), and elastomer polydimethylsiloxane (PDMS) into CNT NP within a few seconds, followed by polymerization. A systematic review was conducted to report on the NP/polymer nanocomposite fabrication process and the properties of the nanocomposites together with their detailed exemplary applications.