The joints in the precast segmental ultra-high performance concrete（UHPC） bridge are quite critical for the mechanical performance of this type of bridge，especially for its shear performance. Based on the Plastic Damage Model in ABAQUS，the nonlinear finite element model（FEM） was built to investigate the direct shear behavior of the UHPC dry joints with big shear keys，and this FEM was corrected and verified by the experimental data from available literature. On the basis of the verified FEM，the direct shear behavior and the failure mechanism of the UHPC dry joint with big shear keys were investigated. Two failure modes of the UHPC joint，slip failure and direct shear failure，were found in this paper. Furthermore，a parameter analysis was conducted to the big shear key in the UHPC dry joint，where the considered parameters included the depth，inclination and lateral compressive stress. According to the modeling results，the reasonable big shear key is suggested to have the height to shear height ratio of 0.15~0.25 and the inclination angle smaller than 37°. Finally，based on the Mohr’s stress circle theory and FEM results，a formula for predicting the direct shear capacity of UHPC dry joints was established. It was proved that the formula had promising availabilities by test data.