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 were carried out. During the test, the fatigue loading was suspended at certain intervals for static load testing, and the crack width, mid-span deflection, UHPC surface strain, steel base plate strain and tensile reinforcement strain of the specimens under static load were collected. The test results showed that: With the tensile longitudinal reinforcement rate of more than 1.58% of the flat steel plate - UHPC composite deck plate under the fatigue load (load ratio is 0.25, the maximum cycle load is 0.8 times the static cracking load), its crack width grow slowly with the number of cycles. With a crack width of 0.1 mm as the life standard, its fatigue life reaches or exceeds 16 million cycles; The stiffness of flat steel plate-UHPC composite bridge deck with more than 1.58% of the tensile reinforcement rate 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 no longer controlled by strength, and it should be controlled by the durability that prevent corrosion caused by excessive crack width.