HVDC transmission lines can generate an effect on the environment nearby due to the electric field and the ion current density after the corona occurs, so the calculation of ionic flow field is significant to transmission lines design and electromagnetic analysis. However, there is no effective method to calculate the characteristic parameters of ionic flow field under rainy condition. Based on Deutschs assumption, a calculational method is proposed with considering the effects of raindrops on ionic flow field. In the method, the space-charge-free electric field distortion caused by raindrops is especially considered, and the charged raindrops are seen as a charge background of transmission lines. The field strengths and ion current densities calculated using this method are compared with the experimental results in the published literature, it shows that the method is effective and accurate enough under fair and rainy conditions.