To accurately guide the insulation design of coaxial bus duct of 500 kV GIS, it is necessary to study the dynamic distribution characteristics of electric field caused by structure and different voltage waveform. Taking the L-type and T-type bus bars in 500 kV GIS manufactured by a company as examples, the transient electric field characteristics under very fast transient voltage and lightning wave are studied based on time domain finite difference method. Results show that under very fast transient overvoltage, the peak value of the electric field in the L-type bus is smaller than that of the electric field under steady state, but the reflection of the wave front should be paid attention to. In the T-type bus, the reflection of the electric field is larger than that in the L-type bus. The maximum value of electric field is about 1.13 times as large as the steady-state value, and there is obvious oscillation in the wave front when 4 ns ramp wave overvoltage is applied, so it is needed to use transient electric field to check the insulation design. Under lightning wave, there is no reflection in the electric field of each node in the L-type bus and the T-type bus, and the electric field can be verified by the calculation result of the electrostatic field.