Abstract: To address the bus voltage sag induced by large-power motor starting in the auxiliary power system of third- generation nuclear power units, an electromagnetic transient simulation model of the nuclear plant 6 kV auxiliary power system represented by AP1000 was established. The transient current characteristics and bus voltage sag mechanism during direct starting of large-power induction motors such as reactor coolant pumps were systematically analyzed. Based on the voltage divider principle of the system equivalent circuit, the quantitative relationship between the bus voltage per-unit value and the system short-circuit capacity as well as the motor starting apparent power was derived, yielding the voltage sag depth calculation formula. On this basis, the operating principles and engineering applicability of multiple suppression techniques were comprehensively analyzed, including star-delta reduced-voltage starting, soft starters, variable frequency drive starting, and starting sequence coordination. A comprehensive suppression strategy combining dynamic configuration of static var compensators and multi-motor starting sequence coordination was proposed. Simulation results show that after implementation of the strategy, the voltage sag depth of the 6 kV auxiliary bus decreases from 18.7% to 6.2%, the voltage recovery time is shortened by approximately 43%, and the bus voltages of all important loads are maintained above 0.90 (pu), satisfying the technical specification requirements for stable operation of the nuclear plant auxiliary power system.