To address the fatigue cracking issue and the resultant maintaining difficulties of orthotropic steel bridge decks （OSDs）， which are common in long-span steel bridges， the research group at Hunan University proposed a new UHPC-based strengthening structure and with this strengthening technique， the fatigue cracks in OSDs can be exempt from repairing. Based on a long-span in-service suspension bridge as the research subject， this paper introduces the application background of the new strengthening technique， and presents a series of in-site experimental tests that were performed to reveal the response of the OSD under three states， i.e.， with original the asphalt overlay， with no overlay， and with a UHPC strengthening layer. The fatigue performance of four typical fatigue-prone details in the OSD was systematically revealed based on the test results， including the stress distribution as well as the stress reduction after strengthening. At the same time， a local finite-element （FE） model was established for the bridge and the load cases considered in the experimental tests were simulated in the FE model. It was found that the stress response surfaces of the fatigue-prone details are basically consistent with the corresponding test results， and the maximum difference is about 10%. The research results show that for each fatigue-prone detail， the stresses exhibit no significant differences under two states， i.e.， with the original asphalt overlay and without overlay， indicating that the original asphalt overlay deteriorated seriously and could not effectively improve the mechanical state of the OSD. The stresses of the fatigue-prone detail were reduced by 41%~85% when the OSD was strengthened by a thin UHPC layer. For the fatigue-prone detail of the U rib-to-deck plate welded connection， the stresses were reduced by 85%， while for the fatigue-prone details at the upper cutouts and at the intersections between the U rib and diaphragm， the stresses reductions were revealed as 44% and 41%， respectively. This confirms that the thin UHPC layer could significantly improve the local bending stiffness of the OSD and consequently reduce the stresses caused by vehicle loads. Moreover， the response lines were obtained for different fatigue-prone details in the OSD and it was found that the response lines differ significantly for different fatigue-prone details. For the fatigue-prone detail near the upper cutout， the transverse response scope of the compressive stress is small， while the transverse response scope for the tensile stress is larger. Furthermore， the longitudinal response scope for the fatigue prone detail near the upper cutout is also short. For the intersections between the U rib and diaphragm， the stress is maintained at a high level even though the distance between the vehicle load and the fatigue-prone detail exceeds the spacing of 3 successive diaphragms or cross beams.