In order to study the influence mechanism of different shear key forms on the crack resistance of ultra-high performance concrete (UHPC) composite slabs, flexural performance tests are conducted on one monolithic UHPC slab and three composite slabs with different composite interface configurations (including stirrup and truss shear keys) to systematically analyze the influencing mechanism of structural forms and shear key types on failure modes and crack propagation patterns of the specimens. The results indicate that: 1) UHPC slabs exhibit typical flexural failure characteristics with overall good ductility; 2) Due to secondary loading loading effects, the composite form will reduce the initial cracking load of the specimens, while the influence of structural form (monolithic vs. composite) on failure mode, crack propagation path, and ductility indices remains relatively limited; 3) Shear key reinforcement can significantly improve the cracking load of composite slabs by enhancing the confinement effect in the tensile zone; in the meantime, the shear key reinforcement participating in the force increases the effective cross-sectional area of tensile and compressive reinforcement and further enhances the flexural capacity of composite slabs. Compared to the composite slab without shear keys, the flexural capacities of the stirrup-reinforced composite slabs and truss-reinforced composite slabs increased by 25%~30%, with the truss shear key demonstrating relatively better comprehensive effects in improving both crack resistance and load-bearing capacity. The study findings in this paper can provide theoretical foundations and data support for the optimized design and engineering application of shear keys in UHPC composite slabs.