In the planning of ultra high-voltage transmission lines that crossover southeast regions of China, one of the key questions is the prediction of vibrations in the conditions of a complex terrain, rainfalls and wind. As the presence of raindrops in the wind filed influences the dynamic stability of the transmission line, the objective of this paper is reveal the mechanism of rain-wind induced in-plane vibration by analyzing the effects of time-varying mass of raindrops and the motion of upper rivulet that is suspended to the high-voltage transmission line. A rain-wind induced in-plane vibration model with time-varying mass is proposed base on the finite element method and the central difference method. Applying our method to the Tuo-chang transmission line, we use the finite element method to develop the numerical model of the transmission line movement under the action of wind and corona forces, while time discretization is made with the central difference method. The results show that the time-varying mass of raindrops and moving upper rivulet have obvious effects, and the model enables better comprehension of the rain-wind induced in-plane vibrations of transmission lines.