The invention discloses a variable gap arc discharge experimental device which comprises a fixed electrode base fixedly arranged on the ground to simulate an overhead line tower and a motor base simulating a tree root. A fixed electrode facing the motor base is horizontally arranged at the top end of the fixed electrode base, and a phase voltage is connected into the fixed electrode. A stepping motor is horizontally arranged at the position, perpendicular to the fixed electrode, of the top end of the motor base, an output end shaft of the stepping motor is connected with an eccentric wheel, and a moving electrode which is in the same axis with the fixed electrode in a static state is fixedly arranged at the side edge of the eccentric wheel. During the experiment process, the situation thattrees swing with wind and are connected to overhead lines of a distribution network in an overlap mode in thunderstorm weather can be simulated, the characteristic that gaps between the trees and a bus bar change continuously along with the swing of the trees are truly reflected, and a foundation is provided for the research on occurrence of single-phase fault grounding of a power grid through the trees.