Abstract: Aiming at the engineering faults such as burnt thyristor trigger boards, false conduction of power devices and internal cabinet circulation easily caused by 15 kV instantaneous spike voltage generated during the pressure withstand test of power regulating cabinets for polysilicon reduction power supply, this paper studies ways to improve the operational stability of power supply systems for reduction furnaces. Firstly, the generation mechanism and conduction path of spike voltage as well as the deficiencies of existing suppression schemes are analyzed. A composite suppression system consisting of bidirectional clamping diode for amplitude suppression, contactor for physical isolation and PLC program for intelligent management is innovatively constructed to collaboratively eliminate high-voltage spike interference from three dimensions: spike amplitude limiting, conduction path blocking and working condition interlock control. The selection of protective components, hardware layout and interlock control logic based on Siemens S7-300 PLC are completed, and the effectiveness of the proposed scheme is verified through industrial field tests. The measured results show that the composite scheme can stably clamp the original 15 kV spike voltage within a safe range of 3.8–4.2 kV. The average monthly equipment failure frequency drops from 2 to zero, the annual equipment maintenance cost is reduced by 262,800 CNY, and the production continuity is increased by 30%. The proposed scheme features small reconstruction workload, no production shutdown required during construction, and strong adaptability to industrial sites with high temperature and severe electromagnetic interference. It can provide an engineering reference for overvoltage protection of similar high-voltage power electronic systems in the photovoltaic polysilicon industry.