The invention discloses a flue gas desulfurization system which comprises a pulping system, an absorption tower and a gypsum dewatering system which are sequentially communicated, wherein a filter screen box is arranged between the pulping system and the absorption tower and/or between the absorption tower and the gypsum dewatering system and comprises a box body and a filter screen arranged in the box body; the box body comprises a working side and a cleaning side which are mutually communicated; the filter screen can move between the working side and the cleaning side; a top plate at the working side is provided with a material inlet; a bottom plate at the working side is provided with a material outlet; and the top of the cleaning side is provided with a cleaning port. Since the filter screen in the flue gas desulfurization system can move between the working side and the cleaning side of the filter screen box, on-line cleaning can be performed on the filter screen without influencing the normal operation of the flue gas desulfurization system.