Focused on device security issue caused by overheat of indoor substations main transformer convective heat transfer process in summer, a convective heat transfer ventilation with side air inlet is proposed in this paper. Based on computational fluid dynamics (CFD), the numerical simulation and analysis results show that the heat dissipation performance of indoor substation installed in this special ventilation is very excellent.By using this system, the temperature of main transformer could be reduced. Both sides of main transformer could be cool down in a very short time due to the cooling fin, which solves the problem of less air motion right above the transformer. The whole air flow field tends to be proportional and reasonable. This kind of ventilation ensures the security and stability of indoor substation working in summer.