采用数值模拟法和有限元网格细化法对比给出电场的合理参考范围，分别选取距离阴极三结合点(0.005)(21/2d)处(d是绝缘体的厚度)和距离阳极三结合点(0.008 5)(21/2d)处的电场强度为参考点。通过优化均压环形状，采用阴极激发闪络和阳极激发闪络两种模型来控制三结合点处的场强，优化得出阴极三结合点处场强值为2.55 kV/mm，阳极三结合点处场强值为23 kV/mm。

The theories of secondary electron emission avalanche and anode-initiated flashover model are applied to optimizing the insulator stack grading rings. All results are calculated through the finite element method. Two methods are taken into account, one is to fit numerical results with analytic results, and the other is to refine those elements. Through comparing the results from the two methods, reasonable values of electric field intensity are confirmed. As for cathode, the value is the electric field at a distance of (0.005)(21/2d) from the junction (d is the thickness of the insulator), and for anode, it is the electric field at a distance of (0.008 5)(21/2d) from the junction. After the optimization, 5 bumps are selected for 10 grading rings. The largest electric field in the cathode triple junction reduces to 2.55 kV/mm, while that in the anode triple junction reduces to 23 kV/mm.