The capacitance and resistance to the ground are important parameters of transmission lines, eg. the capacitance may affect overvoltage and relay protection of transmission lines. Both may change when corona occurs, especially for UHVAC transmission lines at rainy days. In this paper, a corona simulation model of AC transmission lines is established from which space charge distribution and total radial current varying with time can be obtained. And then dynamic capacitance is abstracted from simulation results directly. Next, Fourier decomposition is used to obtain equivalent parameters of UHVAC transmission lines with corona. Through simulation of a single-phase 1000 kV UHVAC transmission line under different roughness coefficients which are used to represent the effect of different rainfall, the influence of rainfall on the capacitance and resistance of the transmission lines is given. The calculation results can provide a reference for the overvoltage research and relay protection design.