First, a primary fluoric ion concentration detection process is performed, and a primary calcium salt addition process is performed to add calcium salt in a first reaction tank, wherein the dosage of the calcium salt in the primary calcium salt addition process is determined according to the detected fluoric ion concentration. Thereupon, a secondary calcium salt addition process is performed to add calcium salt into the second reaction tank. Following that, a solid-liquid separation process is performed to separate calcium fluoride from the wastewater, and a secondary fluoric ion concentration detection process is performed upon the wastewater after the calcium fluoride is separated. Finally, the dosage of the calcium salt in the secondary calcium salt addition process is determined in a feed back control manner according to the detected fluoric ion concentration.