The invention relates to a flue gas desulfurization waste liquid treatment and activated sludge reduction treatment method, belonging to the field of environmental protection. Excess activated sludge from a sewage treatment plant is introduced into a flue gas desulfurization waste liquid oxidation treatment device. Sulfite in wastewater is converted into sulfate under the action of the activated sludge, and meanwhile, the activated sludge is in the endogenous respiration state, and can acquire energy by self-consumption, thereby implementing activated sludge reduction. The invention can be implemented only by laying one pipeline and introducing a right amount of excess activated sludge from the sewage treatment plant into the flue gas desulfurization waste liquid oxidation device; and therefore, the technique is simple, and is especially suitable for enterprises in which a sewage treatment plant and a flue gas desulfurization facility are simultaneously constructed.