Taking a proposed self-anchored suspension bridge with steel-UHPC composite girder as the engineering background, the spatial finite element member structure model of the whole bridge is established. The influence of UHPC bridge deck pouring stage and UHPC bridge deck segmental pouring scheme on the mechanical performance of the stiffening girder in the construction process of "first cable-stayed and then suspension" is studied. The research results show that the mechanical properties of UHPC bridge deck and steel beam are better when the bridge deck is poured after the completion of temporary cable-stayed bridge than those after the completion of suspender tension and after the completion of transforming the cable-stayed and cable-suspended system under the final completion state of the bridge. For UHPC bridge deck poured after the completion of temporary cable-stayed bridge, it is firstly to pour the beam segment in the stayed cable zone, and then to pour the beam segment near the central bearing point. Under the final completion stage of the bridge, the mechanical properties of mid-span bridge deck and steel beam are better than those of firstly pouring the beam segment near central bearing point and then pouring the beam segment in stayed cable zone.