Circulating fluidized bed coal-fired boilers that adopt conventional measures of improving desulfurization efficiency exhibit difficulty in achieving ultralow SO2 emission. To solve this problem, our team originally proposed a desulfurization process of limestone injection into furnace with desulfurization of activated carbon (AC) in the rear. In this process, most of the SO2 in furnace is initially removed. The rest is transported to the AC desulfurization device and adsorbed by AC. The utilized AC is then thermally regenerated. The SO2 released by thermal regeneration is sent back to the furnace and is ultimately removed by limestone. AC adsorption and desorption experiments are conducted in this study. Furthermore, the parameters of the process are optimized, and the economic feasibility of the process is assessed. Compared with that of two-stage desulfurization, the cost of the proposed process is higher but still affordable. Therefore, the process is economically affordable and technologically feasible.