A gutter shaping combined steel formwork of a reinforced concrete building structure comprises a gutter steel formwork body used in cooperation with an existing shaping combined steel formwork of a national standard. The formwork body is a long-strip-shaped groove profile board and comprises a bottom board, a left side board, a right side board and a top edge of the formwork body. One side board is arranged perpendicular to the bottom board and the other side board inclines outwards relative to the bottom board. The top edge is parallel to the bottom board. Connecting holes are formed in the portion, on the side board perpendicular to the bottom board, of the top edge at intervals. An inner supporting piece is arranged between the left side board and the right side board which are tightly attached to the bottom face of the top edge. Two connector boards are symmetrically arranged on the inner sides of the two axial ends of the steel formwork body. A plurality of inserting holes are symmetrically formed in the two connector boards. Two adjacent formwork bodies are axially connected through inserting pieces penetrating through the inserting holes of the connector boards. The shaping formwork is reusable, form erection and form removal are convenient, wood resources are saved, engineering cost is reduced, and the labor productivity and the construction quality are improved.