In frozen days, natural draft dry cooling system (NDDCS) faces a serious freezing risk, so it is of great concern to propose anti-freezing measures. In this work, the coupled thermo-flow models of NDDCS with steam condenser in a typical 600 MW power plant are developed, and the anti-freezing circulating water flow rate and opening degree of louvers are simultaneously calculated for air-cooled sectors under cold ambient conditions. At not very low ambient temperatures, the anti-freezing of NDDCS can be reached by increasing water flow rate alone. Besides, at -10 degrees C in the absence of winds, the minimum water flow rate of 16.84 kg/s can prevent NDDCS from freezing. But as the ambient temperature drops further, the louvers should be closed down synchronously. At the low ambient temperature of -30 degrees C, the louvers are switched down with the opening degree of 55% in the absence of winds, 16% and 15% at the wind speeds of 4 m/s and 16 m/s respectively for the frontal sector, besides the anti-freezing water flow rate is kept highest as 50.505 kg/s. If the operating turbine back pressure is higher than the anti-freezing back pressure, NDDCS can be free from freezing in cold winter. (C) 2017 Elsevier Ltd. All rights reserved.