Because of the high moisture content within sludge, it is difficult to obtain a sludge dewatering rate of >60% within municipal sludge treatment. In this study, the semi-dry flue gas desulfurization residue was added as a sludge conditioner to concentrated sludge under microwave radiation. The results showed that with a gradually increased dosage of the residue to the sludge, the settling velocity of the sludge increased considerably and moisture content in the filtered cake and the specific resistance to filtration value decreased considerably. After treatment by the residue conditioning combined with microwave radiation, the sludge settling performance and filtration dewaterability were further improved. The optimal operation conditions were summarized as a mass ratio of the residue and sludge by a dry weight basis of about 1:1 w/w, and 50 s microwave radiation. The addition of the residue can also speed up the filter cake drying process, effectively kill harmful microorganisms, and promote toxic metal passivation. These improvements may be attributed to the alkalinity of the residue and microwave radiation damaging the stability of biological cells within the sludge, while the CaSO 4 · 2 H 2 O and CaSO 3 · 0.5 H 2 O acts as skeleton formers, thereby facilitating the sludge dewatering process.