Abstract: To investigate the influence of a floating conductor on the breakdown characteristics of air gap, tests of 80cm rod-plane gap discharge with conductors of different arrangements were conducted. The influences of the diameter and position of floating conductor on the breakdown voltage were studied. Meanwhile, the discharge process was recorded by using a high-speed camera and a mirror. The results show that a large diameter conductor with diameter-to- length ratio greater than 0.6 arranged in the middle of the gap can raise the breakdown voltage and lead to bypassing phenomenon. The possible reasons for "bypassing" discharge were discussed using recorded images and electric field simulation. It is pointed out that the weak electric field around the large diameter floating conductor, free charge accumulation and the hindrance to photo ionization are the important reasons for the bypassing phenomenon.