In order to reduce bird damage and improve the reliability of power system, it is necessary to study the bird steamer flashovers. A cotton strip impregnated with the bird streamers simulating liquid was used to simulate bird streamers in this experiment. And the I-type composite insulator string was used as the research object. During the test, the relationship between the spatial position and the corona onset voltage and the breakdown voltage was studied when the bird streamers fell from directly above and to the side of the grading ring. Studies have shown that when the distance between the bird streamers simulation strip and the grading ring is getting larger and larger, the breakdown voltage is getting higher and higher. The breakdown field strength gradually decreases as the gap distance increases, the value is close to the breakdown electric field intensity with the rod-rod gap. Finally, the simulation model is established by COMSOL. The experimental and simulation results provide a reference for the construction and prevention of transmission lines.