<p>Reinforced thermoplastic pipes (RTPs) are extensively utilized in oil and gas transportation systems. The stability of the polymer structure and performance during service is critical to ensuring the safe operation of the pipes. In this study, visual inspection, thermogravimetric analysis, tensile testing, Vicat softening temperature measurements, pressure testing, differential scanning calorimetry, x-ray diffraction, and gel permeation chromatography were employed to evaluate the structural and performance stability of polymers in RTP after actual service. After four years in operation, the hardness and VST of the liner pipe decreased by 17.0% and 4.2%, respectively. Mechanical properties, including elongation at break, yield strength, and modulus, declined by 50.1%, 12.1%, and 41.2%, respectively, indicating a reduction in both stiffness and toughness. The performance degradation is primarily attributed to swelling initiated by the transported medium, leading to a noticeable reduction in crystallinity (19.3%) and orientation (26.1%) in the liner pipe. The swelling had a minimal effect on the condensed state structure of the polyester fibers, preserving their crystallinity and orientation, which resulted in only minor degradation of the tensile strength of the reinforcement fibers. The outer protective layer, which had little to no contact with the transport medium, maintained its structural integrity and performance over time. For oil transmission conditions, the liner material should be made of polymer materials with strong molecular structure polarity, strong intermolecular forces or cross-linked structure, in order to reduce the effect of medium swelling on the mechanical properties of the lining material.</p>