The finite element model of the overhead conductors was established and the modal analysis was carried out with the ANSYS finite element analysis software. The results showed that the natural frequencies of various vibration modes of the overhead conductors decreased with the icing thickness increasing. A method for measuring the natural frequency of overhead conductors based on the phase-sensitive optical time domain reflectometer （Φ-OTDR） was proposed， with which the on-line monitoring of overhead transmission line icing could be realized. A transmission line icing simulation device was built. The test results of a commercial Φ-OTDR showed that the small-amplitude vibration frequency of the suspended line was only related to system properties， not external excitation. The natural frequency of the suspended line decreased with the increasing number of the coating layers， which showed excellent monotonic relationship. It was testified that this method could provide a reliable solution for on-line monitoring of overhead transmission line icing.