An　open　design　scheme　is　presented　for　medium　and　small　hydropower　plant　automation　system.　Its　system　structure　and　software　design　are　introduced.　The　PLCs　(Programmable　Logic　Controller)　of　local　control　unit　and　the　protection　devices　are　connected　directly　with　Ethernet.　IEC60870-5-104　protocol　and　OPC　(Ole　for　Process　Control)　are　used　in　communication.　Technologies　of　OOP,　multi-procedure,　multi-thread　and　COM　are　used　in　the　design　of　supervisory　and　control　software.　The　identification　of　system　data,　the　implementation　of　OPC　server　accessing　and　the　application　experiences　of　IEC60870-5-104　protocol　are　described　in　detail.　The　system　runs　reliably　with　better　standardization　and　openness.