Abstract: The paper analyzed requirements of automatic control system for water purification plant in Shendong Mining Area, constructed automatic control system involving host computer subsystem, PLC subsystem, instrument detection subsystem and electrical control subsystem. It particularly described hardware constitution, software design of host computer and PLC of the automatic control system. The practical application shows that the automatic control system achieves automatic control of all process stages of water treatment, ensures operating stability and reliability of water treatment system, and has characteristics of simple operation and high degree of automation.