In order to address the challenges of structural performance and maintenance efficiency in large-diameter shield tunnels within soft soil stratum, an innovative PMPW (precast middle partition wall) design was investigated through theoretical analysis and numerical simulation. A three-dimensional structural model was established using finite element software, and the mechanical behavior evolution under surcharge conditions was systematically analyzed throughout the "pre-contact" to "post-contact" phases. The results show that a 6.88 cm top gap in the PMPW design was found to effectively control the flexible-rigid transition of the structure. The interface between the top wedge segment and adjacent segments was identified as most susceptible to damage under the combination of low axial force (3 000 ~ 4 000 kN) and high bending moment (approximately 800 kN·m). Arc components were observed to be more prone to initial damage compared to middle partition walls. A performance enhancement scheme incorporating local UHPC (ultra-high performance concrete) reinforcement was proposed and tested. It is concluded that the proposed scheme could improve the deformation resistance of precast components by 20% ~ 60%, providing theoretical foundation and engineering guidance for the design and construction of large-diameter shield tunnels in complex geological conditions.