This study investigated the effect of phosphoric acid treatment on carbon fibers to enhance their flame retardancy, the impact of carbon fiber surface treatment conditions on the interfacial adhesion between carbon fibers and PA6, and the chemical reaction mechanisms on the carbon fiber surface. Phosphoric acid treatment resulted in a flame-retardant effect, with a limited oxygen index of over 52.8%, and V0 level flame retardancy characteristics in the UL-94 test when the concentration exceeded 0.5 vol.%. When the treatment time was fixed at 30 min, the tensile strength increased by approximately 2%, and the interfacial shear strength (IFSS) increased by approximately 11% at 0.5 vol.% phosphoric acid, accompanied by an increase in hydroxyl (C-O), carbonyl (C=O), phosphate (P-O), and phosphoryl (P=O) groups. However, at concentrations higher than 0.5 vol.%, the tensile strength decreased by approximately 90%, and the IFSS decreased by approximately 12%, compared to the untreated nonwoven fabric. When the treatment time was varied at 0.5 vol.% phosphoric acid, both the tensile strength and IFSS increased continuously up to 10 min, with a 31% increase in tensile strength and a 17% increase in IFSS, along with an increase in O=C-O, P-O, and P=O groups, as well as surface energy. After 10 min, the tensile strength decreased by approximately 20%, while the IFSS and surface energy remained relatively unchanged.