Abstract: In the electric deflection of electron beam，the causes of bright spots and the factors of spots length are studied theoretically. The total expression x，of electron flow deflection and the length Δ，of bright spots are obtained and simulated numerically. The results show that bright spots are caused by the conical electron beam introduced into deflection field ( the angle Δθ，of conical electron beam is very small) ，and the reason of Δ varying with the change of accelerating voltage U1 ，deflection voltage Ux is Δθ asymmetry distribution on the central axis of deflection field. Also，Δ depends on the value of Δθ.

Key words: electron beam, electric deflection, spots length