Under rain-wind conditions, rivulets often form on the surface of transmission lines, which may cause the rain-wind induced vibration. This kind of vibration seriously threats the safety and stability of the transmission tower-line system. The rivulet’s effect on the original cross-section formation of the transmission lines and its movement are likely to be the cause of the aerodynamic instability. To analyze the mechanism of rain-wind induced vibration on transmission, the transient aerodynamic characteristics of rain-wind induced vibration on transmission line is studied based on the hybrid substructure approach in this paper. The transmission line (including the rivulet) and the flow field are considered as two substructures; the vibration response of the transmission line is measured through wind tunnel tests treated as the moving boundary condition of transmission line in the flow field. Only the flow field around the moving boundary is simulated by the computational fluid dynamics code to obtain the aerodynamic characteristics of rain-wind induced vibration on transmission line. Lastly, the transient aerodynamic lift and drag coefficients are applied to the transmission line, and the comparison is carried out between the vibration responses of the transmission line and experimental results. The research results show that the hybrid substructure approach is an effective approach to obtain the transient aerodynamic characteristics of rain-wind induced vibration on transmission line, and also provides an effective method to obtain aerodynamic characteristics which are inconvenient to be measured in the tests.