To investigate the flexural performance of commonly used segmental joints in shield tunnel construction in large-diameter tunnels under ultra-high water pressure, this paper relies on a large-diameter underwater tunnel project. Using ABAQUS software, a refined three-dimensional numerical model of shield tunnel segments with different joint forms is established. Through numerical simulation, the mechanical properties and damage development mechanisms of three types of typical structural segmental joints are studied. The results show that: The flexural performance of the three types of structural segmental joints shows nonlinear and staged characteristics under different working conditions, and high axial force loads effectively improve the flexural performance of the joints. The development of the flexural performance of segmental joints is divided into multiple stages. Under positive bending moments, it is divided into four stages. Under negative bending moments, double-layer internal and external joints are divided into four stages, while single-layer joints and double-layer external joints are divided into two stages. There are significant differences in the flexural performance of segmental joints with different structural forms. Under positive bending moments, the flexural bearing capacity of joints is in the order of double-layer internal and external > single-layer joint > double-layer external, and under negative bending moments, it is completely the opposite. Under ultra-high water pressure loads, damage to segmental joints occurs at the waterproof sealing grooves and bolt holes. Cracks at the inner waterproof grooves of double-layer internal and external joints will increase the leakage path and affect the structural bearing capacity and waterproof performance. There are great differences in the damage development modes of joints with different structural forms. Under positive bending moments, the double-layer external joint form is the least favorable for resisting positive bending moment loads. Under negative bending moments, it has the highest structural bearing capacity and is suitable for being arranged in negative bending moment areas such as tunnel shoulders.