A method for detecting an abnormal traffic on a control system protocol, includes: checking whether session information exists in a management table; adding a new entry to the management table; checking whether a transaction ID in a table entry is the same as that of the received MODBUS request message; and checking whether data and length thereof of the received MODBUS request message are the same as those in the table entry. Further, the method includes detecting an abnormal traffic; and updating the table entry with packet information of the MODBUS request message.