Abstract The insulation design of a medium-voltage cascaded energy storage system is very important due to its direct access to the AC medium-voltage network. This paper focuses on improving the reliability and safety of the system operation, and realizes the insulation design of a 10 kV medium-voltage cascaded energy storage system through the selection of insulation materials and structural innovation. In the structure, high insulation sheet molding plastic was used to suspend the battery. The ultra-low conductivity coolant was used to enhance the insulation performance between battery modules. The stability and practicability of the insulation structure are demonstrated by effective test methods. The research shows that the medium voltage insulation design meets the insulation requirements of the 10 kV voltage class, and has high industrial practical value for the project of medium voltage cascaded energy storage system.