The intelligent device which transmits data to the upper computer by serial communication is needed in many industrial control occasions. When using the nonprocedure protocol Mitsubishi Q-series PLC only supports receiving "fixed length" and "variable length fixed end" data objects, which can not match the intelligent devices those who send "variable length fixed end" data properly. For this reason, a free serial communication method of Q series PLC is designed. By using sequence control program, this method realizes "variable length without fixed end" data reception, abnormal data elimination and self-recovery of communication module. Such method has the characteristics of breaking the limitation of communication module itself and making self-defining communication protocol available. Experiments show that it can support multiple intelligent devices at the same time and has good adaptability andversatility.