Aiming at secondary combination device cabin of prefabricated cabin in smart substation, and combining rack-mounted secondary device project, this paper researches characteristics of secondary combination device of prefabricated cabin. The cabin structure, operating environment control, internal space of the cabin are optimized. Design of rack construction and cabin structure are integrated. Design of efficient heat dissipation and work environment in cabin is optimized. Design of efficient heat dissipation and ventilate in cabin is optimized. Through a case study of 220 kV Substation in Yunxian County, an optimized design scheme of secondary combination device cabin environment is introduced. High temperature heat simulation result analysis in prefabricated cabin verifies its good heat dispersion.