Efficient and accurate calculation of power frequency electric fields in transmission lines in the design phase is of great significance for enhancing the safety of power systems. The Charge Simulation Method (CSM) is a widely used approach in this direction. However, existing CSM-based approaches suffer from oversimplified transmission line models, pronounced end effects, and limited accuracies. To address these limitations, this paper proposes an improved model for three-dimensional electric field calculation in transmission lines based on CSM, utilizing a Competitive Swarm Optimizer (CSO) algorithm to enhance the model accuracy. Firstly, the proposed model discretizes the catenary into segmented line charges as the fundamental charge units to improve the modeling fidelity. Secondly, based on the high-dimensional characteristics of the model, a Large-Scale Global Optimization (LSGO) problem is further formulated, and the CSO algorithm is used to solve the optimal arrangement of the image line charges. Finally, the total spatial electric field is synthesized from the charge distributions. Experimental results indicate that the proposed method improves the computational accuracy when applying CSM to power-frequency electric field analysis in transmission lines.