In ultra-deep wells, tubing strings are prone to failures such as breakage, corrosion, and cracking under high-temperature, high-pressure, and highly corrosive conditions. A novel quantitative risk assessment model for tubing string failures in ultra-deep wells is proposed in this study. First, risk factors for tubing string failures during the production phase are thoroughly analyzed based on failure cases and production data from oil and gas wells in a specific oilfield. Second, to overcome the limitations of single-weighting methods, which involve subjective bias or excessive dependence on data characteristics, the Analytic Hierarchy Process (AHP) combined with the Entropy Weighting Method is innovatively adopted to determine the weights of tubing string risk factors. The Fuzzy Comprehensive Evaluation Method integrated with the Similarity Clustering Method is used to scientifically calculate the risk values of tubing string factors. Finally, the proposed model is applied to the risk assessment of Well Y1. Key risk factors and their risk levels are determined. The results show that quantitative risk assessment can effectively reduce the risk of safety accidents and provide a technical basis for the scientific management of ultra-deep well tubing strings.