This study investigates the effect of prestress tensioning in the negative moment region on the mechanical behavior of flange plate wet joints, using an under-construction simply-supported-to-continuous T-beam bridge as a case study. Through construction process monitoring and parametric numerical simulations, the stress distribution patterns in the wet joints were analyzed. For construction convenience, the referenced project adopted the sequence of tensioning prestressing tendons first, followed by casting longitudinal wet joints. The parametric analysis considered two additional scenarios beyond the original construction sequence (Method 1): casting wet joints before tensioning prestress (Method 2), and partially casting wet joints in the negative moment region before tensioning prestress, followed by casting the remaining wet joints (Method 3). Monitoring results showed that while the actual prestress tensioning levels varied by over 10% during construction, the wet joints remained largely unaffected due to post-tension casting. After completion of wet joint casting, transverse stresses were predominantly compressive, with longitudinal tensile stresses only observed near transverse beams at low levels (maximum value: 15 × 10^-6). Monitoring data indicated that prestress tensioning did not directly increase the cracking risk of wet joints. Parametric analysis demonstrated that all three construction methods provided certain stress margins for wet joints, with maximum principal tensile stresses under permanent loads measuring 0.3, 1.5 and 0.9 MPa respectively. The first two methods showed continuous significant tensile stress development after superimposed dead load application, while the third method (staged casting of wet joints) maintained low tensile stresses even after superimposed dead load, though it involved more complex construction procedures. These findings provide valuable references for mechanical analysis and process optimization in similar structural construction projects.