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The main branch of 500kV high-voltage DC circuit-breaker (HVDC CB) adopts a series structure of multiple fast mechanical switches, which requires an effective distance within 2ms and can withstand over-voltage. Therefore, higher requirements on the dynamic voltage balancing and synchronization characteristics are put forward. In this paper, the action characteristics of 500kV HVDC CB and the stress of fast mechanical switches in main branch are analyzed. A model of fast mechanical switches in 500kV HVDC CB is built, and the distributed capacitance parameters are extracted. The voltage equalization measures of multi-switch series structure are studied. It is suggested that the voltage balancing capacitance can be 65nF, the damping resistance can be 200Ω, and the voltage balancing resistance can be 200MΩ. The voltage distribution of fast mechanical switches in different periods of operation is explored under operation lightning impulse (OLI), and the requirement of synchronization in the case of loss of redundancy is explored. The interval should not exceed 0.17ms.