This article introduces the static scheme, the construction method and the seismic-resistant designing of the Fluvià Viaduct, which has a total length of 835m (45+10x60+2x70+50m) and an average height of 18m. So, it is the longest viaduct in the Barcelona-Madrid-French Border section between Barcelona and France. The spans of this structure are medium-long for prestressed concrete solutions in Spain. The general concept of the structure is a 14 span continuous post-tensioning prestressed concrete main girder, simply supported in piers and abutments, with a useful width of 14m. The main girder cross-section is a 4.0m depth one-cell box with quasi-vertical webs, which results in an close-to-optimum length to depth ratio of L/15 for the typical spans. In piers P-11; P-12 and P-13 depth grows linearly up to 5.5m in order to adapt properly to the bending moments appearing in longer spans. The main girder is prestressed to one of the abutments, which acts as fixed point of the structure for supporting horizontal longitudinal actions like traction-braking forces and earthquake loads. With a fixed end in one abutment, the viaduct has a total expanding length of 835m, which is far from the maximum expansion length reached in others structures in Spain, but nearly close to the recommended maximum length that can be solved with conventional track devices. Earthquake loads are the limiting factor in the design of most of the elements of the substructure (foundations, piers and abutments, and diaphragms). The construction method for the piers and the abutments is the most common for this sort of structural parts, and the greatest engineering effort is done to design the girder for a span-by-span construction with a self-bearing falsework designed initially for road bridges of 50m of maximum spans. The use of this available self-bearing falsework for the construction of a railway viaduct (with a cross-section heavier than a highway cross-section for the same span length) deals directly with most of the limitations that influenced the design and construction of the girder. It was necessary to limit the load on the falsework girders to an allowable limit, that prevented from casting the whole cross-section at once. Constructionengineering of the Madrid-Barcelona-French Border high speed railway viaductover river Fluvià. So every phase of the sequential construction method (every “span”) has two sub-phases: 1.- Only bottom slabs, webs and outer flanges are cast. After hardening of concrete, prestressed up to 60%. In this sub-phase, a self-bearing structure is achieved, so falsework can be launched while works go on in the current “span”. In this phase the cross-section is temporarily opened, so it is necessary to close it with steel struts that can be reused.