The invention relates to a limestone-
gypsum wet desulphurization of 
coal-fired boiler 
flue gas, and in particular relates to an out 
tower double circulation desulfurization 
system and a desulfurization process thereof; the out 
tower double circulation desulfurization 
system comprises a 
tower body, an out tower low pH value 
slurry pool and an out tower high pH value 
slurry pool; the bottom of the tower body is provided with a 
flue gas inlet, the top of the tower body is provided with a 
flue gas outlet, and a low pH value spray zone, a conical liquid collecting bucket, a high pH value spray zone and a 
demister are arranged fixedly in turn from bottom to top at the position between the 
flue gas inlet and the 
flue gas outlet in the tower body. The out tower low pH value 
slurry pool and the out tower high pH value slurry pool are arranged outside the tower body, so that operation effect is visual, the problems such as bubbling, overflowing, 
agitator seal position slurry leakage and the like of a traditional desulfurization 
system with a slurry pond arranged in an absorption tower can be effectively avoided, the 
failure rate of the desulfurization system is reduced, the desulfurization equipment availability can be improved, the normal 
unit operation can be ensured, full oxidation and efficient desulfurization can be ensured, and high-quality 
gypsum can be produced.