The power system belongs to the energy supply system. Its safety is extremely important. As an important part of the power system, transmission tower structures are often damaged by earthquakes or the subsidence caused by earthquakes. In this paper, seismic action and subsidence are coupled into an overall mechanism through a physical simulation. A 500 kV transmission tower was used as the study object. Based on incremental dynamic analysis (IDA), the seismic responses of the structure are obtained by changing the intensity of earthquakes and subsidence separately. A probabilistic seismic fragility analysis model is established based on four seismic damage limit states: slight damage, moderate damage, severe damage, and collapse. Relative intensity of earthquakes and subsidence determines which dominates the structural damage. This study provides theoretical explorations and practical analytical methods for the damage study of transmission towers in the case of earthquake-seismic subsidence coupling.