Generally, in serial communication, the transmission of protocol data frames on the line will be interfered by various kinds of errors. Its reliability is poor, and its Error-tolerant and error-correcting ability is low. However, the serial communication protocol can ensure the high reliable transmission of protocol data frames on the serial communication lines. It has been used in some equipment and has been fully implemented in practice. This paper describes the protocol data frame format of the serial communication protocol. The protocol data frame format includes the start flag field, the length field, the sequence number field, the address field, the application data field, the CRC16 check field and the end flag field. This protocol data frame format design ensures that the application data can be transmitted reliably in the line. This paper also describes the framing, receive, send and analysis of the protocol data frame, and judges and processes the loss of the protocol data frame, which greatly improves the reliability, real-time and fault-tolerance of the serial communication protocol.