Abstract: Ultra-high-voltage transmission lines are typically long, and various types of over-voltage issues are prominent. The installation of parallel high-voltage reactors to suppress over-voltages is a conventional design scheme. Under conditions of single line-to-ground faults or two-phase short-circuit faults, long-distance transmission lines equipped with parallel high reactors are prone to power-frequency resonance over-voltage issues due to improper matching of reactance and capacitance parameters. By conducting in-depth research on the mechanisms of power frequency resonance over-voltage, this paper proposes effective mitigation strategies and provides a method for calculating the parameters of the small neutral point reactor of parallel high-voltage reactors based on engineering practice. Ultimately, the correctness of the theoretical analysis and parameter calculations was verified through simulation, providing strong guidance value for the over-voltage design of ultra-high-voltage power transmission projects.