Abstract: The cooling efficiency of high-temperature slab in the secondary cooling zone of continuous casters depends on whether the droplets can penetrate the vapor and liquid films. This process is governed by the droplet dynamic parameters （velocity, size, and quantity）, which is closely related to the nozzle structure and process parameters. The internal-mixing air-mist nozzle of secondary cooling zone was taken as the research object, the cold state and hot state experiments were combined, the spray characteristics and the heat transfer mechanism on the slab surface under different process parameters were investigated, and the influence mechanism of droplet characteristics on the heat flux of slab surface was revealed. The results show that during the air-mist cooling process, as the temperature of slab surface decreases from 1 000 ℃ to 900 ℃, the surface heat flux exhibits a gradual increase, with the rate of increase diminishing progressively. Under different operating conditions, the local water flow density, droplet vertical velocity component and Sauter mean diameter on the slab surface exhibit a lateral decrease from the center toward the edges, resulting in a significantly enhanced heat transfer intensity at the center than at the transverse position 100 mm away from the center. An increase in spray height from 120 mm to 160 mm leads to a notable reduction in local water flow density, droplet vertical velocity component, and Sauter mean diameter at the center of slab surface. Consequently, the average cooling rate decreases from 22.3 ℃/s to 9.2 ℃/s, accompanied by a decline in the increase rate of average heat flux from 3 251.9 W/（m²·℃） to 2 175.0 W/（m²·℃）. As the air pressure increases from 0.2 MPa to 0.4 MPa, the main reasons contributing to the rise in the heat flux at the center of slab surface are the increase in the vertical velocity component of droplets and the decrease in the Sauter mean diameter of droplets. However, as the water pressure increases from 0.4 MPa to 0.6 MPa, the predominant factor contributing to the rise in heat flux is the increase in local water flow density. The average heat flux at the center of slab surface exhibits a positive correlation with the local water flow density and droplet velocity, while demonstrating a negative correlation with droplet size. The findings of this study provide experimental evidence and theoretical basis for optimizing spray parameters in the secondary cooling zone of continuous casters, establishing a regulation mechanism of cooling intensity on the slab surface for high-speed casting, and enhancing slab quality.