Based on a mass work of structure design and optimization about the casting enclosure in high voltage switchgear, some structure design principles were offered. Some typical structures had been simulated and optimized, using finite element analysis. The technology reason in choosing base thickness of the enclosure was identified; the intensity problems root in whole structure and partial area were compared; importance of homogeneous deformation was concluded; method of structure design and optimization was offered. At last, test pressure putted on casting enclosure, effectiveness of the design principles were verified, and practicability of optimization method was proved.