On December 18, 2023, Ms6.2 Jishishan earthquake occurred in Gansu Province, China. In order to explore the cause of the earthquake damage, oil leakage occurred in the transformer casing of a 750 kV substation. The refined finite element simulation models of two kinds of transformer-casing systems were established, and the seismic response analysis was carried out. The results show that for the transformer-casing system, due to the significant difference in stiffness between the transition flange and the upper and lower part of the connection, the stiffness of the connection is weak, and the difference in corner displacement is too large, the bolts are deformed, and the sealing rubber gasket loses its sealing property and leaks oil. Compared with the glue-mounted connection casing, the buckle connection casing is weaker. Although it is conducive to dissipating energy to avoid structural damage, the displacement response is more significant. The design of the transition flange should consider the influence of the transformer riser and the stiffness of the sleeve.