Abstract: With CFX software, a two-dimensional model of a flow field is built to study on the flow past two circular cylinders in a tandem arrangement by using the finite volume method. Firstly, the upstream and downstream cylinder Strouhal in different spacing are analyzed at Re=200. Through the comparison between the calculation results and previous literature results, the computation in this paper is proved reliable. Then the changing characteristics of lift and drag coefficients of the upstream and downstream cylinders in different spacing are researched. It is obtained that the critical distance of the flow between two circular cylinders in tandem is 3.375D~3.5D at Re= 200. Finally, some conclusions are gained though the changes in the flow field in different spacing: when the distance between the upstream and downstream cylinders is less than the critical distance, vortex shedding of the upstream cylinder does not exist; when the distance between the upstream and downstream cylinders is more than the critical distance, vortex shedding of the upstream cylinder occurs; the phenomenon of vortex shedding of downstream cylinder always exists. The results of the research provide a theoretical basis for the development of computational fluid dynamics and Aerodynamics.