Distributed safety-critical real-time systems are required to be real-time and reliable. By the use of CSMA/CD mechanism, Ethernet is not suitable to provide real-time and reliable service. To solve this problem, this paper presents a hard real-time communication protocol based on shared Ethernet and which can provides schedulability analysis and on-line scheduling based on TDMA mechanism with master/slave scheme. And it is oriented to message, guarantees hard real-time communication. This protocol schedules not only the transmission of the messages but also the production of the messages, and supports dynamic changes in the communication requirements. This protocol resolves the unpredictability problem of Ethernet communication, in order to improve the real-time and reliability of the system, which doesnpsilat modify the hardware of Ethernet.