Abstract: Four-circuit transmission lines play an important role in addressing the issue of reducing the occupation of land resources and improving the transmission capacity per area units. This paper establishes the three-dimensional model of the tangent tower of 500 kV four-circuit transmission lines, and studies the voltage-sharing characteristics of the upper insulator grading ring by using the COMSOL multiphysics. Firstly, the voltage-sharing characteristics of the upper insulator with and without a grading ring are compared. The results show that with the small grading ring, the maximum field strength on the surface of the upper insulator is reduced by 25.9%. Then, the parameter optimization designs of the small rings of the upper insulators on the semi-composite cross arm and the full composite cross arm are carried out respectively. The combined parameters of the ring diameter and pipe diameter of the small ring are analyzed, and the single value analysis of the lifting distance is carried out. Finally, the optimal parameter configuration of the small rings of the upper insulators of the semi-composite cross arm and the full composite cross arm are obtained. The results show that the size of the grading ring required by the semi-composite cross arm is larger than that of the full composite cross arm. The installation of the optimized grading ring on the upper insulator further reduces the surface field strength of the insulator, and the surface field strength of the grading ring and the composite insulator are also kept within the control ranges.