Abstract: In order to further study the mechanical properties of the flat steel plate-UHPC composite bridge deck under fatigue load, according to the bridge deck design of an ultra-long-span composite cable-stayed bridge, a ultra-high cycle fatigue tests with negative moment of two full-scale models with different reinforcement ratios (1.69% and 1.86%) were carried out. During the test, the variation patterns of crack width, stiffness, UHPC strain, tensile reinforcement strain, and steel plate strain of the specimens with the number of loading cycles were investigated. The test results showed that: Using a crack width of 0.1 mm as the service life criterion, under fatigue loading conditions with a load ratio R of 0.25 and a load level S of 0.8 or greater, the crack width in the flat steel plate-UHPC composite bridge deck increases slowly as the number of cycles increases, and the fatigue life is no less than 16 million cycles.; The stiffness of the flat steel plate-UHPC composite bridge deck decreased slowly with the increase of fatigue cycles, while the ultimate bearing capacity under hogging moment almost no degradation; The restraining effect of tensile reinforcement on the UHPC restrict the propagation of cracks along the depth direction, making a region of possible fatigue pull-out of steel fibers between the UHPC surface and the top longitudinal reinforcement. When the depth of this area was limited, it’s influence on the degradation of the fatigue performance and residual bearing capacity of flat steel plate-UHPC composite bridge deck under hogging moment can be ignored. The fatigue design of the flat steel plate-UHPC composite bridge deck should be controlled by the durability that prevent corrosion caused by excessive crack width.