In order to better reduce SOx emission, improve the efficiency of the desulfurization system of thermal power unit, reduce the operating cost of the desulfurization system, and improve the economic benefit of thermal power plant, the limestone/lime-gypsum wet flue gas desulfurization system of a 630 MW coal-fired thermal power unit in a power generation company in Hunan was taken as the research object. The influence of change of sulfur content in fire coal on desulfurization efficiency, operating cost of desulfurization and additional income of desulfurization was tested and analyzed with the rated load, so as to study the economic effect of change of sulfur content in fire coal on desulfurization system. The research results show that when the sulfur content infire coal changes within the range of 0.6%~1.2%, with the increase of the sulfur content in fire coal, the desulfurization efficiency of the system also improves gradually. For every 0.1% increase of the sulfur content in fire coal, the comprehensive benefit of the desulfurization system is reduced by about 1.62 yuan when 1 t coal is burned out. However, when the sulfur content in fire coal is more than 4.0%, the desulfurization cost will exceed the profit, which is uneconomical.