Flow control is an attractive topic in consideration of its significance associated with drag reduction or vibration suppression in fluid-structure interaction (FSI) system. A circular cylinder with a flexible plate in the wake region has been investigated numerically. An immersed smoothed point interpolation method (IS-PIM) has been employed to solve this FSI system. The splitter plate with the same length as the cylinder diameter is placed horizontally in the wake region. The drag variation of the circular cylinder and the behavior of the vorticities have been studied with regard to different locations of the detached plate. A comparative study of the rigid plate control has been presented. It shows that the effect of the flexible plate control is inferior to the rigid plate control because the flow field of the former is more complicated than the latter. When the flexible plate is far from the cylinder relatively, it would produce an opposite effect.