During the breaking process, the erosion of anode contact greatly affects the result of high current vacuum breaking. Therefore, it is important to study the forming and transforming process of anode modes for improving the breaking performance and life of vacuum circuit breaker. The cup axial magnetic field (AMF) 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 of cup AMF contacts under high current density, the 40 mm diameter cup AMF contacts with six slots is observed and studied by experimental means under the gap distance of 6 mm and 10 mm. In this paper, the arc current-voltage characteristics, vacuum arc shape and surface erosion of contact are analyzed. The analysis results show that the larger the contact gap, the larger the arc voltage, the smaller the arc anode mode threshold current and the more serious the contact erosion degree.