Abstract: In order to improve the economy of the peak cooling system of the direct coal fired air-cooling unit,an optimization calculation method of cooling ratio of the peak cooling system is proposed. Based on the requirements of EPC bidding documents,the condensate volume of the peak cooling system is determined,and the optimal cooling ratio of the peak cooling system with surface condenser is discussed in particular. Studies show that the optimal cooling ratio of“surface condenser + mechanical tower+ circulating pump ”peak cooling system under the designed condenser volume and the designed temperature（35 ℃）is 35. With the reduction of environmental temperature,the optimal cooling ratio at the corresponding temperature increases. For 600 MW supercritical direct air-cooling generator set,when the peak condensate capacity is 280 t/h,the annual income is RMB 8,869,200 after the cost of water resources is deducted.