The effect of the addition method of thermoplastic poly ethylene-co-meth acrylic acid (EMAA) on the interlayer toughness of carbon fiber reinforced polymer (CFRP) composites was studied using a combination of experimental and simulation methods. Firstly, EMAA was made into thin filaments with diameters of 1.2 mm and 50–75 μm, and then added to CFRP composite materials by sewing and layering, respectively. The effects of EMAA addition method and content on the type I interlayer fracture toughness and self-healing efficiency of CFRP composite materials were studied by adjusting the suture spacing and layering density. The results show that the addition of EMAA significantly increased the type I interlayer fracture toughness of CFRP composite materials, with improvements of 110% and 402% achieved through stitching and layering, respectively. In addition, numerical analysis using the cohesive zone model (CZM) further demonstrated that EMAA can enhance the type I interlayer fracture toughness of CFRP composite materials. After thermal healing, double cantilever beam (DCB) tests were conducted again, and it was found that the interlayer toughness of CFRP composite materials toughened by suture and layup was effectively restored, with repair efficiencies reaching 85% and 105%.