Abstract: With the application of 330 kV EHV cables in northwest power grid,it is important to investigate the characteristics with their impact of transient switching overvoltage in EHV cable lines. Firstly,the simulation model of typical 330 kV cable line is built in electromagnetic transients program,and the amplitude variation of the overvoltage in overhead line and the cable line is calculated,showing that the overvoltage amplitude of the cable is relatively lower than that one of the overhead line. Then the propagation and dominant frequency characteristics of the overvoltage are analyzed with the application of return circuit theory in modal theory. Under the same conditions,the dominant frequency of the overvoltage in the cable is lower,and the propagation mode voltage decays seriously,affecting the maximum value of the overvoltage. Finally,simulation results show that the voltage stability of the outlet cable is affected during the electro-magnetic transients in 330 kV cable lines.