Aerodynamic drag generated by high-speed trains significantly affects trains' operating cost and restricts a further increase in speed. Currently

the drag reduction method with streamlined bodies has reached its limit. The presence of external insulation equipment disrupts the roof's smoothness

making it an important source of aerodynamic drag. However

roof equipment also assumes an essential role of electrical insulation

so both aerodynamic characteristics and insulation performance need to be considered. Therefore

a drag reduction method based on the non-smooth structure is proposed for the roof insulator

in which aerodynamic characteristics on and around the insulator are changed by constructing dimpled surfaces on the sheds. In this study

parameters of dimple structure with an excellent drag reduction effect were determined

achieving a drag reduction rate of 17.51%. Simulations and experiments were conducted to verify that the dimpled insulator exhibits good electrical insulation performance. Finally

a mechanism analysis of drag reduction with a dimpled structure was provided in terms of velocity vector

turbulent kinetic energy and turbulent dissipation rate. This study provides guidance for drag reduction of external insulation equipment and energy savings of high-speed trains.