The invention discloses a fabricated steel and concrete composite beam structure and an implementation method thereof. The fabricated steel and concrete composite beam structure disclosed by the invention is formed by a concrete beam, steel and concrete combined sections and short steel beams; the concrete beam consists of concrete, reinforcements, drilling devices and prestressed cables; each short steel beam consists of section steel, stiffening ribs and end plates; each steel and concrete combined section is formed by shear keys, concrete, stiffening ribs, an anchorage system, reinforcements, end plates, an upper steel flange plate and a lower steel flange plate; the steel and concrete combined sections are transition sections between the concrete beam and the short steel beams; the concrete and steel flanges of the steel and concrete combined sections are connected by the shear keys to bear an external force together. The prestressed cables are arranged in the concrete beam and the steel and concrete combined sections; the corresponding end plates are arranged at both ends of the short steel beams; the short steel beams are connected with a steel and concrete combined column in a bolt or welding mode. According to the fabricated steel and concrete composite beam structure and the implementation method thereof, which are disclosed by the invention, rapid fabricating construction is adopted, so that cost is reduced; full-factory prefabrication is adopted, so that component quality is ensured; excellent properties of two materials of steel and the concrete are sufficiently played, so that mechanical performance of the structure is improved.