The service performance of seamless expansion joints made of polyurethane (PU) material for bridge structures was studied using full-scale tests and finite element models. Firstthree modified PU materials with different mix ratios were prepared. The initial setting timetensile strengthtear strengthhardnessroad adhesionwater absorption rateresistance to ruttingand aging performance were tested and compared to determine the optimal PU mix ratio for seamless expansion joints for bridges. Thenfour full-scale seamless expansion joint specimens were designed and fabricated using the selected PU materialand a corresponding finite element model was also developed. Finallyby comparing the experimental measurements and the finite element calculationsit is shown that the designed seamless expansion joint exhibits excellent service performance under uniaxial tension/compressionvertical deformationand low-cycle fatigue loading conditions. Additionallythe research results indicate that setting a 45° inclination angle between the modified PU seamless expansion joint and the road surface could significantly improve the stress state and prevent early cracking at the joint.