Abstract: To address the risk of power-frequency resonance arising from transmission line reconstruction at ultra-high-voltage (UHV) converter stations, this study investigates an optimization method for selecting the parameters of neutral grounding reactors with small reactance. Taking a 500 kV power transmission project as a case study, the influence of the neutral small-reactance parameters of high-voltage shunt reactors on power-frequency resonant overvoltages is analyzed, with particular emphasis on the reactance configuration of the lines connected to the L converter station after reconstruction. An optimization approach based on the full compensation principle combined with engineering margin considerations is proposed, and the impact of transmission line length deviations on resonance risk is systematically evaluated by calculating power-frequency resonant overvoltage levels under different line lengths and reactor configuration scenarios. The results demonstrate that appropriate selection of the neutral small reactance effectively suppresses resonant overvoltages and ensures stable system operation, providing a theoretical basis for shunt reactor parameter configuration and practical guidance for enhancing power system stability.