Abstract: In order to fully use of resources and reduce desulfurization cost, a large quantity of magnesium slag generated from the traditional technology was used to produce magnesium by Pidgeon process as the base. And we utilized the filtering generated powder dust in magnesium production gyration kiln dust wiper and magnesium production raw material dolomite as accessories and mixed at a certain ratio to manufacture the desulfurization agent for wet desulfurization,and researched the desulfurization effect of magnesium slag, powder dust, mineral in single, and mixed with each other and together as the desulfurization agent. The results proved that, the best desulfurization effect was achieved when magnesium slag:powder dust:mineral=10:0.3:0.035(mass ratio), then the combined desulfurization agent water solution (10.335 g/100 mL) could consume 67.58 mL dilute sulphuric acid with concentration of 1 mL/L. Alkalinity activate agent (Na₂CO₃, NaOH) was also added for further improving the desulfurization effect. The results showed that NaOH had better activate effect, then the combine desulfurization agen could consume 214.42 mL of dilute sulphuric acid when add 2% NaOH, which was 3.17 times of that before activation. SEM results showed that reaction area of composite desulfurization agent obviously increased after adding activate agent. The activated magnesium slag based composite desulfurization agent could be used for wet desulfurization of flue gas, which could realize the resource utilization and achieve the purpose of treating waste with waste, also have a good market prospect.