The main control building of the substation is not only to meet the operational requirements of precision equipment, but also to meet the working requirements of personnel. The design of external shading should take into account the requirements of energy saving and daylight. In this study, a typical substation building as an example in the hot-summer and warm-winter area was chosen. The change of indoor light environment and energy consumption of this building in different shading facilities were simulated and analyzed by using energy and lighting simulation software. And the suitable range of size of the shading facilities were obtained by taking the building energy efficiency and lighting coefficient standard value as evaluation index. Finally, it is clear that the size design of the external shading of this kind of building should adopt the method of double goals optimization of energy saving and natural lighting, to change the way that outside shading coefficient as a single evaluation parameter.