The lime-limestone powder is added to water to make a slurry, as an absorbent with a pump into the absorption tower and full contact and mixing with the flue gas, sulfur dioxide in the flue gas and calcium hydroxide in the slurry and the air drummed in from the lower part of the tower for oxidation reaction to generate calcium sulfate, calcium sulfite, after reaching a certain saturation, discharge the absorption tower, and then concentrate and dehydrate to make its water content less than 10%, crystallize to form a mixture of gypsum dihydrate and calcium sulfite. It is then sent to the gypsum silo for stacking with a conveyor.