The Second Dongting Lake (SDTL) Bridge is the longest steel truss girder suspension bridge in China, which has a main span of 1480 m. To alleviate deterioration issues in the orthotropic steel deck (OSD), the steel-ultra high performance concrete (UHPC) lightweight composite deck (LWCD) was proposed for the SDTL Bridge. The LWCD is composed of a 12 mm OSD and a 45-mm UHPC layer, with the two components connected through headed studs. As the first time of LWCD to be used in the long-span flexible bridge, the constructional method of casting UHPC was carefully developed. Considering that the area of the bridge deck is about 65000 m2, the UHPC layer was divided in to 12 zones and the zones were cast step-by-step. Water tanks were deployed on the bridge deck prior to casting UHPC, and during the casting of each UHPC zone, the corresponding water tanks within the current zone were removed. By doing so, the deflection and rotation of the truss girder as well as the tensile stress in UHPC could be controllable. This paper validate the feasibility of the proposed construction method by Finite-element analysis. The maximum torsion angles of the truss girder in each construction stage are less than 0.60°. In addition, the strains of UHPC were recorded during construction to analyse the stress in the UHPC layer. The in-site test indicated that the longitudinal and transverse strain of the UHPC layer were stabilized at -50 µε ~ 50µε and -20 µε ~ -120 µε after steam curing respectively, which have no cracking risk. Thus, the studies in this paper reveal that the proposed constructional method should be suitable in the construction of UHPC on long-span flexible bridges.