The invention discloses a flue gas desulfurizer and a flue gas dust collection, desulfurization and wastewater treatment method. The flue gas desulfurizer comprises a flue gas emission zone, a defogging zone, a tower disc zone, a spraying zone and a wastewater treatment zone from top down in sequence, wherein the wastewater treatment zone is divided into an oxidation flocculation zone and a circulated clear liquid zone by a vertical partition plate I; the circulated clear liquid zone is completely isolated from the oxidation flocculation zone and the spraying zone through a partition plate IIat the top of the circulated clear liquid zone; the vertical partition plate I is provided with a filtering medium; a flue gas pipeline II extends into the oxidation flocculation zone; a plurality groups of flue gas branch tubes are sequentially and horizontally distributed on a flue gas pipeline II from the bottom up inside the oxidation flocculation zone. By adopting the flue gas desulfurizer, flue gas dust collection, desulfurization and wastewater treatment are carried out in one tower, so that the floor space can be greatly reduced, expense of device construction and transformation can bereduced, and device operation cost can be also reduced.