To accurately guide the insulation design of 500 kV disconnector room of GIS, it is necessary to study the dynamic distribution characteristics of electric field caused by structure and voltage waveform. The transient electric field characteristics of 500 kV disconnector room under very fast transient overvoltage (VFTO) and lightning wave are studied based on time domain finite difference method. Results show that no matter the disconnector is closed or opened, high frequency oscillations will occur in the electric field of each node, moreover, the electric field distribution will change constantly under VFTO. Oscillations have a high amplitude under 4 ns and 20 ns wavefront voltage time, which may seriously threaten equipment insulation. Especially, when the 4 ns ramp wave voltage is applied, the maximum value of electric field is about four~five times of the steady-state value and the oscillation frequency is higher than that of coaxial bus duct. So transient electric field must be calculated to check the insulation design. Under lightning wave, electric field of each node in the disconnector is almost non-reflex, and can be verified by the calculation result of electrostatic field.