Abstract: The purpose of this paper is to present an improveddomain decomposition method, and to simulate the flow around a singlecylinder and two cylinders in tandem arrangement. For the calculations offlow around a single cylinder, the size of steady vortex at low Re here wasclose to the results obtained from experiments. For the calculations of flowaround two cylinders, research was applied to discuss the lift coefficient,drag coefficient and the frequency of lift curve in various spacing. Theresults we obtained in this paper are consistent with experiments, andprovide us with some basic knowledge about vortex-induced vibration.