During the cooling process of the flat steel under the insulation cover, the cold air flowing in from the gaps of the cooling bed has an excessive flow rate, causing the cooling speed of the flat steel at this location to be too fast. This results in an uneven cooling rate of the flat steel, making it prone to bending and having an excessively high hardness, which reduces the product quality of the flat steel. Therefore, the chimney effect ducts and mechanical ventilation structure were proposed to reduce the air velocity at the gaps of the cooling bed and improve the uniformity of the temperature field of the flat steel. The air temperature field and flow field of the cooling bed were simulated using the Ansys Fluent software. Based on the simulation results, the effects of three chimney effect structures and a mechanical exhaust device on the air flow velocity at the gap and on the temperature distribution of the flat steel were analyzed. According to the research results, it is found that the air velocity at the gap of the cooling bed directly affects the temperature difference (ΔT) between the flat steel at the gap and non-gap locations, and the larger the air velocity, the larger the ΔT. The four proposed improvement schemes can effectively reduce the air velocity at the gap and decrease ΔT, among which the small pipe hot air structure based on the chimney effect performs the best, which can reduce the air velocity at the gap by 73.8% and ΔT by 47.73%. In addition, this structure can utilize the waste heat to form a natural chimney effect, which has significant advantages in terms of energy saving and environmental protection.