The low-viscosity ε-caprolactam resin exhibits rapid infiltration in thermoplastic resin transfer molding (T-RTM), causing an unstable flow front at a macroscopic level and uneven impregnation between inter-/intra-tows. Uneven impregnation arises when there is a substantial difference between external and capillary pressures, resulting in the formation of micro-/macrovoids in the fiber preform. To address this issue, we propose to minimize total void content by regulating key parameters (contact angle between resin and carbon fiber, surface tension, viscosity, and resin infiltration viscosity) related to capillary pressure. Unlike previous studies focused on optimizing impregnation velocity through external pressure control, our approach employs polyamide 6 oligomers to enhance the compatibility between ε-caprolactam resin and carbon fiber, reducing the contact angle and minimizing void content by managing capillary pressure.