An optical network monitoring system and method thereof are proposed. An optical line terminal (OLT) transmits a first optical signal to a plurality of optical interference devices. After the first optical signal passes the optical interference devices, the optical interference devices reflect back a plurality of second optical signals corresponding to the optical interference devices respectively to the optical line terminal. The second optical signals have different optical path differences. An optical/electrical converter unit converts each of the second optical signals into an electrical signal. A spectrum analyzing unit analyzes the electrical signal to extract a frequency component of the electrical signal, thus the fiber connection status to each optical network unit in the optical network system could be obtained. Therefore, the purpose of monitoring the optical network system is achieved.