Abstract: In recent years, the development and application of fiber-reinforced thermoplastic resin matrix composites with environmentally friendly and high toughness characteristics have received considerable attention. The fiber/matrix interface bonding properties of thermoplastic composites play an important role in load transfer. However, the water immersion environment can easily cause fiber/matrix interface damage, which seriously threatens its service safety. This article intends to introduce multi-walled carbon nanotube (MWCNT) into the fiber/matrix interface phase in order to explore the in-situ monitoring and self-healing of the interface damage of the thermoplastic composite before and after water immersion. Firstly, the preparation method of the MWCNT interface sensor with the best electrical conductivity is experimentally investigated to determine the optimal process parameters and introduce the MWCNT into the fiber/matrix interface. Secondly, the samples are immersed in water at different temperatures to investigate the water absorption behavior of the interface phase. Thirdly, the fiber bundle pull-out experiment under the quasi-static loading condition is carried out to in-situ monitor the failure process of the interface phase under the pull-out load according to the resistance change of MWCNT sensor. Finally, the interface damage formed during the pull-out process is repaired by in-situ melting using the applied voltage. The results show that the method used in this paper can obtain a multifunctional thermoplastic composite fiber/matrix interface phase that integrates water absorption resistance, in-situ monitoring, and in-situ repair functions. The proposed method provides guidance for the safety of thermoplastic composites in the service of water-immersed environment.