The abnormal closure of the ethylene feed valve in polyethylene plant caused disturbances in polymerization unit, and emergency measures were taken to avoid the shutdown of the plant. This article uses root cause analysis to investigate the factors causing abnormal valve operation, and ultimately determines that the insufficient power supply of the control system failing to meet the needs of on-site equipment is the cause. The accident potential is eliminated by taking corrective measures, and considerations for the power supply design of control system are proposed.