Recently, the demand for lightweight and high mechanical performance materials have been increasing in various industries. Among them, polyamide 6 (PA6)–based carbon fiber reinforced thermoplastic (CFRTP) has attracted attention as promising structural components, owing to their high impact resistance, superior specific strength, and recyclability. However, the high melt viscosity and intrinsic moisture absorption of PA6 lead to significant challenges in composite processing and long-term stability. To address these issues, thermoplastic resin transfer molding (T-RTM) was employed, in which ultra-low-viscosity lactam monomers were impregnated into carbon fiber preforms and in situ polymerized. Furthermore, ω-laurolactam was incorporated to form a PA6/PA12 copolymer with ε-caprolactam, enhancing moisture stability of the matrix. In addition, tunable mechanical performance was successfully achieved by adjusting the monomer ratios.