Abstract: In order to provide theoretical support for further research on arc control methods, this paper mainly studies the characteristics of free-burning vacuum arc under different gap distance-to-diameter ratio (g/D). A CuCr50 plate contact with a diameter of 42mm is selected in this paper. The experiments are conducted under the conditions of gap distance of 12 mm (g/D=0.286), 24 mm (g/D=0.571), 36 mm (g/D=0.857) and current peak (I_peak) from 2.4 to 5.2 kA. The results show that the cathode spot expansion speed and average carrying current are close under different g/D. Cathode spot expansion speed is proportional to the square root of the peak current. When I_peak > 4 kA, at low g/D (g/D=0.286), there are three anode modes: diffuse arc mode, foot-point mode and anode spot mode. At high g/D (g/D=0.571, 0.857), numerous cathode spots remain on the side of cathode contact after the initial expansion stage, and it is difficult to form stable anode spot on the surface of anode contact. There are great differences in arc characteristics and anode phenomenon between free-burning vacuum arc under high g/D and low g/D. Serious ablative to the side of contact caused by vacuum arc under high g/D must be considered.