In order to study the complex combustion situation in the continuous annealing furnace of a steel hot-dip galvanizing unit, this paper takes the continuous annealing furnace as the research object and constructs a three-dimensional numerical model covering the whole section. The model takes into account the multi-physical coupling effects such as turbulent flow, component transport, chemical reaction and radiation heat transfer in the furnace, and systematically simulates the combustion process of the gas in the furnace and the influence of the air-coal ratio on the temperature and atmosphere in the furnace. By adjusting the parameter of air-coal ratio, it is found that a moderate increase in the air-coal ratio (up to 4.4) has almost no effect on the temperature distribution in the furnace, but reduces the hydrogen escape rate in the direct-fire heating section, and saves as much as 16.16% of the gas consumption compared with the original working condition (3.7), which is a remarkable energy-saving effect on the premise of maintaining the reductive atmosphere. This study provides theoretical support and technical reference for the energy-saving operation and process atmosphere regulation of the continuous annealing furnace in this steel mill.