Metro substation automation system(MSAS) manages many intelligent electronic devices(IEDs) and performs online control, data acquisition, information retrieving and communication with remote control center(CC). A MSAS based real-time embedded system is designed in this paper. It consists of station management level, network communication level and field level. In network communication level an embedded realtime PC-104 system with 2 10Mbps fiber-ethernet ports is employed as network communication device(NCD). NCDs provide different field-bus interfaces to IEDs.In MSAS many policiess have been adopted to optimize its real-time response ability, reliability and flexibility. Firstly, optical fiber media is used for ethernet connection. Secondly, no matter network-bridge PC or embedded PC-104 system adopt real-time operating system QNX. Thirdly, publishsubscribe architecture instead of client-server is adopted to