Setting up an economizer flue gas bypass system is an effective approach to increase the inlet flue gas temperature of the denitration system in a coal-fired power plant and maintain high De-NO_x efficiency under all working conditions. In order to analyze the economy of the denitration system in a coal-fired power plant during frequent load cycling processes, the dynamic models of a 660 MW ultrasupercritical coal-fired power plant and the SCR system are developed. Then, the cost of urea, the environmental cost of NO_x emission and the cost of coal are obtained and compared under different flue gas bypass baffle opening during the loading up processes from 50%THA to 100%THA. The results show that the change value of coal cost is the largest, and the total cost has a certain increase when the economizer flue gas bypass baffle is opened during the load cycling processes. When the flue gas bypass baffle opening is 5% and 10%, the cost of coal increases by ￥600 and ￥1200 respectively during the loading up processes from 50% to 75%THA. Meanwhile, the cost of urea decreased by ￥442.48 and ￥554.89 respectively, while the environmental cost of NO_x emission changed less, only ￥62.22 and￥63.19. Moreover, the cost of coal increased by ￥1800 and ￥2300 respectively during the loading up processes from 75% to 100%THA, and the amounts of change in urea cost and NO_x emission environmental cost were little. Therefore, it is not recommended for coal-fired power plant to open the flue gas baffle during the loading up processes from 75% to 100%THA.