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The flow around two tandem circular cylinders was studied by a three-dimensional numerical simulation of the Navier-Stokes equations at Re=220. The improved virtual boundary method was applied to model the no-slip boundary condition of the cylinders. The results show that as the spacing ratio L/D ≥4, the three dimensionality occurs in the wake. When L/D≤3.5 the wake keeps a two-dimensional state at the Reynolds number Re=220. The critical spacing for the appearance of three-dimensional instability obtained is at the range 3.5＜L/D＜4, similar to the critical spacing found in two-dimensional case. Two sources of instability from upstream and downstream cylinder generate a complicated vortex structures in the wake, investigated by streamlines topology analysis in the streamwise plane. Many other interesting problems were also addressed in this paper.