Abstract: In order to solve the insulation design problem of 750 kV cross-linked polyethylene cables, a design method for insulation thickness of extra-high voltage AC cable was presented. By analyzing switching impulse and lightning impulse withstand voltage level of insulation, insulation thickness of 750 kV cables was studied and calculated in combination with test data, and the type test and margin test were carried out on trial-made 750 kV cable products. Results showed that the design value of insulation thickness in this paper could provide reference for manufacture of 750 kV cross-linked polyethylene cables and provide technical support for 750 kV cable projects.