Abstract: In view of problems of low production efficiency and bad products quality existed in manual control used by silica production system of one company, the paper proposed a reform scheme of automatic control system of silica reactor based on S7-200 PLC. It analyzed technology process of silica production system and introduced design of hardware and software of the reformed automatic control system in details. The actual application showed that the reformed automatic control system realizes automatic control for flow of acid, alkali and water, automatic control for solution temperature and solution pH value, which improves products quality and production efficiency.