To study the static bending mechanical properties of UHPC （Ultra-High Performance Concrete， UHPC） bridge deck of a super-kilometer hybrid composite beam cable-stayed bridge， UHPC materials with compressive strength greater than 160 MPa and elastic modulus greater than 45 GPa were designed and prepared in this paper， and two full-size UHPC bridge decks with length， width and height of 3.8 m×1.0 m×0.17 m were tested under static load. By analyzing the bending stress state and failure mechanism of UHPC bridge decks， the stress calculation diagram of UHPC bridge deck was proposed， and the cracking moment of UHPC bridge decks and the bearing capacity calculation formula considering the stress reduction of UHPC in the tensile softening section were established. The results show that the UHPC bridge decks have excellent flexural mechanical properties. The average cracking moment reaches 77.4 kN?m， and the average bending moment corresponding to the yield of the tensile longitudinal reinforcement is 237.1 kN?m. The failure mode of the plate is mainly characterized by the yield of the tensile longitudinal reinforcement at the bottom. The calculated values of the theoretical formula are in good agreement with the experimental values. This research work provides a theoretical and experimental basis for the application of UHPC bridge decks in the main girder of kilometer-level cable-stayed bridges.