Bottom die for cast-in-place bridge deck slab support of steel-concrete composite beam and construction method

The invention discloses a bottom die of a steel-concrete composite beam cast-in-place bridge deck slab support and a construction method. The bottom die comprises a plurality of truss floor bearing plates. The truss floor support plates are all arranged below a bridge deck slab needing to be poured, are all prefabricated parts, and each truss floor support plate comprises a bottom plate and a truss structure formed by a plurality of upper chord steel bars, a plurality of lower chord steel bars and a plurality of web member steel bars. A top edge reinforcing steel bar is arranged above the upper chord steel bar; bottom edge reinforcing steel bars are arranged on the outer sides of the two lower chord steel bars below the upper chord steel bars and fixed to the corresponding steel beam flanges through cushion blocks, and at least one bottom edge reinforcing steel bar extends in the length direction and crosses the nearest adjacent box chamber, so that the side beam bridge deck slab cantilever arm section steel