The invention discloses a novel steel structure building system comprising a steel pipe concrete column, a steel main beam, an auxiliary beam, a floor and a wall surface, wherein the floor is an assembled steel bar truss floor support plate, and the auxiliary beam is a precast concrete auxiliary beam; the wall surface is a CCA board and aerated concrete composite lath, the composite lath comprises an upper layer, a core layer and a lower layer, the upper layer and the lower layer are CCA boards, the core layer is an aerated concrete layer, the core layer is covered on the lower layer, and the upper layer is covered on the core layer. The novel steel structure building system disclosed by the invention is accurate in stress, simple in member connection, convenient and quick in construction, convenient for industrial production, beneficial for reducing the environmental pollution and improve the living environment, and good in economical efficiency.