Flashover across the insulator in induction cavity with electron beam load once occurred frequently, which affected the performance parameters and stability of the high voltage high current devices and equipment. This paper studies the vacuum surface flashover across the insulator in the induction cavity of a linear induction accelerator under debugging. After excluding design and fabrication of the insulator as well as cleanness and vacuum degrees, it is found that the electrons from the plasma induced by high energy electrons interacting with the gas adsorbed on the inner cavity wall might play the key role of flashover. These thermal electrons move randomly and some of them will reach and stay on the insulator surface for some time, and flashover happens when high electric field is applied across the insulator with accumulated electrons.