For filling higher input voltage requirement, the input-series system based on attainable power devices is prevalent in industry. Specifically, its auxiliary power supply (APS) is self-powered from submodule (SM) capacitance on medium/high voltage occasions, for easing the common mode noise and insulation problems as well as facilitating the modular design. But in the meanwhile, the APS startup problem is brought into practice. This article points out that the out-of-sync issue of APSs during the startup process is considerable since it causes a severe voltage oscillation in the input-series system with small SM capacitance, which is rarely reported before since this adverse influence is neglectable in the common system with large SM capacitance. Therefore, for the first time, the impact of the parameters on voltage oscillation is assessed in this article, which is based on a half-bridge converter equipped with designed self-powered gate drivers as a target input-series system. After establishing an analytical model, the safety and stability criteria for oscillation are identified, followed by the simulation verification. Further, a two-stage startup scheme and hardware design are proposed to ease the voltage oscillation without the penalty of matching passive component parameters. Finally, the corresponding results from experiments are provided to verify the effectiveness.