Abstract: To identify typical defects and their controlling factors in long-serving mountain pipeline tunnels along the West-East Gas Pipeline, statistical analyses were performed on 20 tunnels in the Jiangxi Section using the professional inspection data collected in 2025. Six major defect categories were evaluated: voids behind lining, concrete strength, reinforcement cover thickness, carbonation depth, cracks, and water seepage, and abnormal surrounding rock. The analysis focused on the correlations among various defects and the response relationship between surrounding rock conditions and tunnel defects. The results indicated that tunnel defects in the Jiangxi Section were characterized by widespread occurrence, localized severe deterioration, and regional differentiation. Cracks, water seepage, and abnormal surrounding rock were the most prevalent issues, whereas voids behind lining, insufficient concrete strength, and substandard reinforcement cover were concentrated in specific tunnels. Marked positive correlations were observed between overall crack frequency and maximum crack frequency per 100 m, as well as between the proportions of wide cracks and reticulated cracks. Conversely, material deterioration indicators exhibited only weak-to-moderate correlations with crack parameters. Finally, fault fracture zones and poor-quality surrounding rock had a significant impact on crack propagation, voids behind lining, and material performance degradation. These findings provide critical technical support for routine inspection and rechecking, risk hot-spot identification, long-term monitoring, and defect remediation in gas pipeline tunnels.