Abstract: Isomerized bluff bodies are commonly found in civil, transportation and Marine engineering. The aerodynamic shapes of each substructure vary greatly, and the shear flow interaction rules are complex. Through pressure test and flow visualization test, this paper studies the aerodynamic and flow characteristics of the rectangular plate-rounded square column system under low turbulence with the gap ratio G/D=0-0.4、the spacing ratio L/D=1.60-5.80 and the Reynolds number Re=4×10⁴-1×10⁵. The results show that, when the gap ratio G/D=0 and 0.1, flow transition occurs first on the shoulder of the windward side; when G/D is increased to 0.3, flow transition occurs first at the lower edge of the windward side. When G/D=0.4, the bistable phenomenon appears on the rounded square column, the flow visualization is represented by the periodic attachment and separation of the leading edge shear flow on the rounded square column surface and it gradually weakens with the increase of L/D, when Re=1×10⁵ and G/D=0-0.4, with the increase of L/D, a quasi-Reynolds number effect similar to the Reynolds number effect of a cylinder appears, and dramatic change occurs in aerodynamic force.