Abstract: The paper provided the design of backup power self-switchover unit based on PLC for the substations of industry and mine enterprises.It has many merits such as flexible control,simple(programme) and convenient maintenance in practice. The design took a miniature powerful relay in output(circuit) and combining special programme to resolve the arc problem of relay contact.This system can largely decrease the cost of operation and(maintenance,) enhance the reliability and economic benefit of substation.