Abstract: The three-dimensional electromagnetic fields simulation of nonuniform transmission lines that were under consideration for the next generation of Z-pinch drivers was performed by a code called CST microwave studio. A coaxial exponential line model was established in which the inside radius of the outer conductor is 100 mm, the characteristic impedance is 0.203 at the input port and 2.16 at the output port and the input voltage is half-sine pulse wave as TEM mode with an angular frequency of 14106 rad/s. A little energy of non-TEM mode is found during the transmission process. The more severely the radius of the line changes, the more energy of non-TEM mode is found. If the length of the transmission line is long enough, the difference of voltage transmission efficiency between electromagnetic fields and circuit simulation will be less than 1%. In this situation, results of electromagnetic fields simulation can be replaced by circuit situation.