<p>The precast multibox girder uses an integral pullout steel inner formwork, which positions the wet joint near the support—the most unfavorable location for shear forces. At this joint, the longitudinal reinforcement ratio is low, no prestressed steel strands pass through, and the prestressing anchor head forms a rigid shear key. To investigate the shear resistance mechanism at the new-to-old concrete interface, static load tests were performed on four groups of eight Z-type direct shear specimens, and numerical analyses were also carried out. A nonlinear contact model, considering the interaction between the rigid shear key and concrete, was developed, and rational tangential and normal constitutive relationships were proposed. The finite element model (FEM) was validated against experimental results, showing good agreement. The effects of concrete strength, shear key diameter, and shear key embedment depth on the interfacial shear performance were systematically investigated, considering principal stress fields, strain fields, and interfacial contact stresses. The results indicate that shear key diameter and concrete strength significantly influence the shear-bearing capacity of the specimens. While embedment depth has a limited impact on the peak load, it helps to expand the bearing area, reduce stress concentration, and improve the specimen’s ductility. An empirical formula for predicting shear-bearing capacity was proposed based on the force mechanism of the rigid shear key specimen, drawing from research on rebar dowel action and shear anchor bolts.</p>