The invention discloses a T-shaped plate girder segment prefabricating unit based on a steel truss and a combined bridge deck slab. The T-shaped plate girder segment prefabricating unit comprises a lower force bearing structure and the combined bridge deck slab arranged on the lower force bearing structure, and the combined bridge deck slab and the lower force bearing structure are both arranged in the longitudinal bridge direction; the lower force bearing structure is a planar truss composed of an upper chord rod, a lower chord rod and a web rod, the upper chord rod comprises a first rectangular steel tube, and the lower chord rod comprises a second rectangular steel tube; the combined bridge deck slab comprises a bottom supporting steel plate, a plurality of upper longitudinal ribs arranged on the bottom supporting steel plate, and a concrete bridge deck slab arranged on the bottom supporting steel plate in a pouring mode, the upper longitudinal ribs are arranged in the concrete bridge deck slab in a pouring mode, and the upper longitudinal ribs are long-strip-shaped steel plates with the middles provided with a plurality of through holes; the bottom supporting steel plate is a rectangular supporting steel plate formed by connecting a left extension steel plate, a right extension steel plate and a first rectangular steel tube. The T-shaped plate girder segment prefabricating unit is simple in structure, reasonable in design, easy and convenient to construct, good in overall mechanical property and capable of improving the structure accuracy and construction convenience of a steel and concrete combined bridge.