The invention discloses a flue gas desulfurizer prepared from a cement hardening slurry in a waste concrete and a method. The flue gas desulfurizer comprises 80-98 parts by weight of modified powder,0-10 parts by weight of quick lime, 0-10 parts by weight of slaked lime, and 0-15 parts by weight of pulverized fuel ash, the modified powder is prepared by pulverizing the waste concrete, separatingto remove a coarse aggregate, obtaining a hardened cement mortar, lightly roasting at the low temperature, grinding, concentrating the powder, separating to remove a silicious fine aggregate, and obtaining the modified powder having the main ingredients of active calcium hydroxide and an amorphous C-S-H gel. The hardened cement mortar is taken as the raw material, a powder material is obtained after the appropriate process treatment, the powder material, as the desulfurizer, can be applied to the removal of SO2 in the high temperature flue gas and the assisted removal of NOX, particularly, theapplication effect of the desulfurizer in a novel dry cement clinker production line is remarkable, and a new through is provided for the full component resource utilization of the waste concrete.