The invention relates to a method for preparing a desulfurization agent from excess activated sludge and mainly solves the problem of environmental pollution in the prior art. The method for preparing the desulfurization agent from the excess activated sludge comprises the following steps: the excess activated sludge and a dehydrating agent are mixed and dehydrated by a press filter until the moisture content is lower than 60%; the dehydrated sludge is pulverized by a pulverizer until the granularity is larger than 0.15 mm and smaller than 1 mm and the specific surface area is larger than 300 m<2>/g, and the desulfurization agent is prepared, wherein the dehydrating agent contains calcium oxide, ferric oxide, aluminum oxide and magnesium oxide, a dehydrating machine adopts a plate-and-frame press filter, a squeezing press filter, a membrane filter press or a chamber press filter. With adoption of the technical scheme, the problem is solved better, and the method can be used for preparing the desulfurization agent.