Abstract: Butt contacts is widely used in vacuum trigger switch and vacuum on-load tap changer. In order to study the vacuum arc characteristics and anode mode change under high current density, the butt contacts with diameters of 15 and 20 mm are tested at 3 and 6 mm gap in this paper, and arc voltage characteristics, arc energy characteristics, arc morphology and contact ablation are analyzed in turn. The results show that, when the peak current is constant, the arc voltage and arc energy decrease with the increase of the ratio of contact diameter to gap distance. The smaller the ratio between the diameter of the contact and the gap distance, the smaller the threshold current of the arc anode mode, and the more serious the ablation of the contact. At the gap distance of 6 mm, the 15 mm contact arc voltage is relatively stable in most of the time, and the anode plume appears before the arc is about to be extinguished, while the 20 mm contact arc voltage fluctuates frequently during the entire arc, the arc mode changes constantly, and the anode plume appears during the entire arc. The research results have theoretical significance for high current vacuum breaking.