In cold winter, natural draft dry cooling system (NDDCS) confronts a severe freezing risk due to high cooling capacity of ambient air. In this work, the anti-freezing of air-cooled heat exchanger by spraying flue gas at the inlet of cooling deltas to pre-heat ambient air is proposed. With the coupled heat transfer models of steam condenser and NDDCS, the cooling performance of air-cooled heat exchanger and the minimum outlet water temperatures of various sectors are obtained at three typical low ambient temperatures, based on which the flue gas spraying strategies for the potential frozen sectors are analyzed. At not very low ambient temperature of 268.15 K, the flue gas should be sprayed for the frontal and middle front sectors at the wind speeds of 8 m/s and 12 m/s. At 263.15 K, the frozen sectors differ greatly as the wind speed increases, besides some non-frozen sectors should also be pre-heated. As the ambient temperature drops to 258.15 K, more flue gas should be sprayed at all wind speeds due to the increased air cooling capacity. By spraying flue gas, the pre-heated air can contribute to the safe and energy efficient operation of NDDCS in frozen days. (C) 2019 Elsevier Ltd. All rights reserved.