Abstract For offsetting the inherent weakness of traditional cooling system, a natural draft hybrid cooling system is introduced to take full advantage of wet and dry cooling technologies, by combining the dry and wet cooling sections in one cooling tower with serial airflow path. The three-dimensional numerical models are developed to evaluate the thermal-flow performance of the hybrid cooling system under different operating and ambient conditions, with the airflow fields and cooling efficiency obtained and analyzed. The results show that the performance of the wet cooling section is superior to the dry one in most cases. The large spraying water flow ratio, low ambient humidity and small wind speed are of benefit to the cooling capacity of the hybrid cooling system, but the cooling efficiency of the hybrid system is more favorable at high ambient humidity and large spraying water flow ratio. At the high ambient humidity, the cooling performance of the dry section in the dry mode can exceed that in the hybrid mode, and the ambient wind even plays a positive role in the wet cooling section with a low spraying water flow ratio. Moreover, the natural draft hybrid cooling system is favorable to the visible plume abatement.