Abstract: In order to further improve the impermeability level of concrete with high impermeability, based on the principle of Slurry Infiltrated Fibrous Concrete, a new method was proposed, which was to in-situ produce a layer of fiber-reinforced cement composite skin on the surface of concrete, to further enhance its resistance to chloride ion penetration. This experiment used different types of concrete for subway construction on site. 78 g/m² non-woven filament fiber cloth was pre-laid on the template to form 150 mm×500 mm×1000 mm specimens, and fresh concrete was poured to infiltrate and wrap the fibers; Demolded after hardening, then the in-situ composite skin layer on the surface of concrete was formed. And the composition and structure of the composite skin layer, as well as its ability to enhance concrete's resistance to chloride ions, were studied through macroscopic and microscopic experimental methods such as concrete chloride ion penetration, ACIS, XRD, and SEM. The experimental results show that the composite skin layer is a long fiber-reinforced cement mortar layer with a thickness of about 1 mm, which is grown on the outer layer of the concrete body with fiber, volume content of about 9%. The composite skin exhibits comprehensive effects including chemically binding chloride ions and significantly reducing the interconnected porosity in concrete. This enables the enhancement of concrete with moderate chloride permeability to a very low permeability level, while extending the chloride diffusion coefficient from meeting the 60-year design lifespan requirements to fulfilling 100-year durability. The composite skin provides a lifelong dense protection for marine reinforced concrete structures, effectively resisting chloride ion penetration.