Regarding the problem of bridge girder's flutter under influence of wind-driven rain, the rainfall similarity laws are firstly established in terms of raindrop impact and surface ponding respectively, according to the characteristics of wind-driven rain and bridge main girder's vibration, besides, the selection principle is also discussed. Then, several typical main girder sections, which are frequently used in long-span bridges, are selected as examples to investigate the influence of wind-driven rain on flutter derivatives and flutter occurrence process by sectional model tests. Results show that there is no significant regularity between the flutter aerodynamic derivatives of the main girder and the rainfall intensity, as the variation of each derivative is almost the same. Comparatively, the variation of derivative caused by rainfall is increased with the wind speed. However, the general variation trend of the flutter aerodynamic derivatives with wind speed is not changed. When the test rain intensity is 120 mm/h, the critical wind speed of the flutter model raises about 20%~30%. Considering the similarity law proposed in this paper, the effect of rainfall on wind-induced flutter characteristics can be ignored.