To investigate the influence mechanism of diaphragm walls on the dynamic response of a segmental underpass tunnel under seismic loading, a three-dimensional finite element model of underpass tunnel in soft soil was established. Seismic responses, including displacement, acceleration, internal forces, and tensile damage of the underpass tunnel structure with and without diaphragm walls, were comparatively analyzed. Results indicate: When the seismic intensity is 0.05 g, the diaphragm walls significantly reduce inter-story displacement in open U-shaped retaining walls, lower peak acceleration at frame''s top and bottom slabs, and markedly mitigate tensile damage to the underpass tunnel, demonstrating overall beneficial effects. When the seismic intensity is 0.215 g, the diaphragm walls significantly amplified inter-story displacement in the shallowest buried segments, markedly increased peak acceleration in most sections of the frame''s top and bottom slabs, substantially elevated shear forces at the top of side walls and axial forces at the top and bottom ends of central columns, and amplified damage at column top and bottom ends, with the detrimental effects of diaphragm walls gradually becoming prominent. The study reveals a dual-mechanism effect of diaphragm walls on seismic responses of underpass tunnel structures, clarifying their differentiated influence on dynamic responses and damage/failure patterns of different structural segments. Findings provide valuable reference for seismic design of similar structures.