In view of the influence of blowing ratio on the impingement characteristics of water droplets in the region behind the hood gap, the relevant criteria for the influence of air film on water droplets are proposed through calculation. It is found that the air film has a blowing effect on water droplets. The water droplet impingement coefficient decreases with the increase of the blowing ratio. The maximum water droplet impingement coefficients at the blowing ratios of 0.24, 0.53 and 1 decrease by 6.73%, 10.95% and 17.11%, respectively, compared with the results at the blowing ratio of 0. According to the shape of the cap, it is found that there are 3 kinds of effects on the water droplet under the influence of the air film, namely, limited blowing, backward blowing and complete blowing, under the arbitrary combination of the 3 variables of blowing ratio, incoming flow velocity and water droplet diameter (V<70 m/s), and the left and right critical water droplet diameter criteria and St criterion are fitted. The experimental results show that the study is helpful to deepen the understanding of the hot air anti-icing of the hood, and has a certain reference value for the anti-icing structure design of the slotted hood.