Snow accumulation on through-wall casing may trigger snow flash accidents on through-wall casing, threatening the safe and stable operation of power system. In this paper, for the snow-covered growth of through-wall casing, firstly, the three-dimensional outflow field and water droplet calculation model of through-wall casing are established, and then the mathematical model of water droplet movement and collision is studied and analyzed. Finally, the numerical simulation of the snow-covered growth of the through-wall casing is performed and compared with the results of the snow-covered growth test of the through-wall casing at Xuefeng Mountain Energy Equipment Safety National Observation and Research Station of Chongqing University, which are in good agreement. This study lays the foundation for the subsequent study of snow cover characteristics and snow cover protection of through-wall casing, and at the same time, it has important reference value for the maintenance of power system security.