Abstract: To investigate the mechanical behavior and failure characteristics of ultra-large diameter shield tunnel segment structures, a three-dimensional refined numerical calculation model of the segment structure based on concrete plastic damage constitutive relations was developed. The study focuses on the distribution of overall vertical displacement, joint deformation, bolt stress, reinforcement stress, crack propagation and distribution, and changes in bearing capacity under ultimate load. The results show that the overall displacement distribution of the segment structure presents a funnel shape. The vertical displacement distribution on both sides of the middle ring is asymmetric. The change in single-point vertical displacement can be divided into two stages: elastic stage and elastoplastic stage, with the largest vertical displacement at the longitudinal joint of the side ring. When the structure fails,the maximum opening of the circumferential joint is 0.075 mm, the maximum misalignment is 0.308 mm, the maximum opening of the longitudinal joint is 4.92 mm, and the maximum misalignment is 3.58 mm. However, the circumferential bolts have not yet yielded, and the bolt stress variation can be divided into three stages: linear decrease, nonlinear increase, and a brief drop. The main failure characteristic of the structure is the first appearance of tensile cracks on the inner arc surface of the middle ring, with cracks more concentrated in the corresponding position of the longitudinal joint. The main reinforcement at the mid-span yields. When the main reinforcement at the midspan yields, the safety factor of the section is 1.383, indicating that using main reinforcement yielding as a criterion for structural failure judgment is both safe and feasible