Based on the immersed boundary concept that the border may be constructed by feedback force, a numerical simulation is carried out by modifying previous inner fluid treatment and incorporating it with non-equidistant grid. Flow around two elongated rectangles in tandem arrangement is computed in the range of Reynolds numbers from 200 to 103. Results indicate that when the Reynolds number is in the range 200–300, a vortex shedding of front rectangle is under control of two separated shear layers. The vortex between the two rectangles belongs to Karman type, which is hindered by small spacings thus symmetric vortices are formed. Shielding effects is mainly reflected by the phenomenon that mean drag coefficients of the rear rectangle is smaller than the front one. At the critical spacing ratio, a vortex sheet between the two rectangles is fully established. The mean drag coefficient also has a jump at this spacing ratio, which is still less than that of the front rectangle. In this phase, as Reynolds number increases, the vortex regime, the jump and the critical spacing all become minimized. When Re=400, the vortex shedding of front rectangle is characterized by an impinging-shear-layer, and thw drag coefficient is no longer a regular oscillation. After that as Reynolds number rises, an impinging-shear-layer is established gradually. More vortices on the surface are produced by flow separation of the front rectangle, which leads to a less magnitude of wake vortex. Shielding effect will disappear at this time. A fluctuation impact on the rear rectangle is induced by drastic vortex shedding from the front rectangle. But proper spacing between the two rectangles can make the drag coefficient of the rear rectangle jump, which is larger than that of the front rectangle.