Abstarct: Abstract Zinc-oxide surge arresters, as one of the most important protective devices in high-voltage networks, play a fundamental role in mitigating transient overvoltages. The reliable and stable performance of these devices is directly dependent on their insulation condition and the uniform distribution of the electric field within their structure. One of the major issues in tall surge arresters with high rated voltage is the concentration of the electric field near the high-voltage terminal. This concentration can lead to corona discharge, increased surface losses, gradual degradation of the insulation and zinc-oxide blocks, higher losses temperature rise in the blocks, and ultimately a reduction in the equipment’s life span. To address this challenge, the use of a corona ring has been widely proposed as a conventional solution. By altering and redistributing the electric field lines, the corona ring reduces the field intensity at critical points and results in a more uniform field distribution along the arrester. However, designing a corona ring in terms of its dimensions and position is inherently complex due to geometric and electrical constraints, as well as the simultaneous influence of multiple design parameters on the electric field. A leading approach in this area is to optimize the corona ring dimensions with the objective of minimizing the maximum electric field generated within the arrester’s column of mextal-oxide blocks. In this thesis, the optimization of corona ring dimensions for a 230-kV surge arrester with a middle terminal is investigated using the Particle Swarm Optimization (PSO) algorithm. To perform the optimization, a co-simulation frxamework between COMSOL and MATLAB is employed. Three parameters—corona ring radius, the tube diameter of the corona ring, and its installation height above the high-voltage terminal—are optimized as the main factors influencing the electric field distribution. The obtained results indicate that using an optimally designed corona ring can reduce the maximum electric field by up to 63.8%, and the percentage variation of the electric field between its maximum and minimum values decreases to 25% with the optimized corona ring. Additionally, increasing the number of middle terminals further reduces the electric field intensity.