[Purposes] To investigate the shear resistance and load transfer mechanism of shear key joints in precast segmental concrete beams. [Methods] Shear resistance tests were conducted on precast segmental beams with three distinct shear key configurations: triple-tooth keys, large-tooth keys, and reinforced large-tooth keys. Based on the test results, a validated finite element model for the shear resistance of dry joints in precast segmental beams was established, and parameter analysis was conducted for different reinforcement configurations. [Findings] Results indicated that for the triple-key specimen, when vertical displacement was applied, the lower shear keys experienced the maximum shear force during the initial stage. Shear key failure initiated at the bottom and propagated upward sequentially. For specimens with large key teeth, the shear area of the shear keys was identical to that of the three-key specimens. However, the large key teeth exhibited superior integrity and more uniform stress distribution. Shear key failure also progressed upward from the root. The stress distribution in the large key teeth specimens with reinforcement was more rational than in those without reinforcement. Adding reinforcement to the three-key teeth increased the shear capacity by 13.34%. In large-tooth specimens, increasing the longitudinal reinforcement ratio boosted shear capacity by 19.01–23.49%, while increasing the stirrup reinforcement ratio enhanced shear capacity by 12.51–13.13%. [Conclusions] Reinforcing key teeth enhances the shear capacity of joint shear keys. In large key teeth shear keys, longitudinal reinforcement contributes significantly more to shear capacity than stirrups. This research aims to provide theoretical support for the design and construction of precast concrete segmental beams.