The mechanical properties of engineering plastics can be enhanced through effective surface mechanical treatment (SMT), which can be applied to various types of engineering plastics, eliminating the limitations of conventional polymer processing and could potentially extend their applications and improve their performance and reliability as structural-functional materials. Inspired by metal forging, this work proposes a simple and effective SMT strategy to enhance the mechanical properties of polyamide 66 (PA66). Tensile tests have shown that SMTed PA66 samples exhibit significant improvements in both Young’s modulus and ultimate tensile strength (UTS), with a 2104 MPa Young’s modulus, almost double that of the pristine samples, and a 35.56% increase in UTS, reaching 183 MPa. Additionally, the modulus within the localized SMTed surface layer could reach up to 14 GPa, which is approximately 14 times higher than that of the pristine sample. The Vickers hardness within the localized SMTed surface layer can be doubled, reaching 10.72 Hv, and the crystallinity can increase by approximately 20% compared to the untreated region. Furthermore, force field molecular dynamics (FFMD) simulations were conducted to investigate the ternary relationship between the SMT method, PA66’s molecular structure, and its properties. The combination of MD simulations and versatile structural characterizations provides evidence that the SMT method’s mechanism, under heat induction, results in a chain-combing procedure that changes the polymer’s molecular morphology microscopically and enhances its mechanical properties macroscopically.