The invention discloses an online monitoring method for a thermal fault of an optical fiber composite overhead ground wire (OPGW). The method comprises the steps that: when a fire or other thermal faults occur along a transmission line where the OPGW cable is located, the ambient temperature of the OPGW rises rapidly, heat is transmitted to a single-mode communication fiber inside the OPGW througha medium such as an aluminum and steel stranded wire coating a distributed optical fiber sensor, the time and amplitude of a backward Raman scattering signal of the single-mode fiber in the OPGW is monitored by a distributed optical fiber temperature measuring device based on the Raman scattering effect of the single mode fiber, at the same time, the thermal resistance value of the aluminum and steel stranded wire of the OPGW is calculated, the location where the fire or thermal faults occur can be determined and monitored and the external ambient temperature can be monitored, and the personnel is quickly guided to handle the fault. By using the Raman scattering principle, the fiber polarization control technology, the noise filtering technology and a thermal resistance value algorithm ofan aluminum and steel stranded wire wrapping layer of the OPGW, the operating ambient temperature monitoring of the OPGW based on the single-mode communication fiber is achieved under the condition of reducing cost and improving monitoring precision.