The parallel-plate Blumlein line generates pulsed square waveform and is widely used in the eld of pulse power technology. This paper nds that the electromagnetic eld leakage of planar transmission line will cause the electric eld enhancement phenomenon in the planar transmission line far from the switch during the pulse formation and transmission of parallel-plate Blumlein line. As the electromagnetic wave propagates to the load end, the electric eld enhancement phenomenon becomes most serious, and which even causes the bulk breakdown of planar transmission line far from the switch. And the breakdown phenomenon caused by the electromagnetic eld leakage has been con rmed by statistics of experimental data. In view of this electric eld enhancement effect, this paper proposes an electrode structure design that can reduce the electric eld enhancement effect, and the simulation result shows that the optimized electrode structure can signi cantly reduce the electric eld enhancement induced by the electromagnetic eld leakage of planar transmission line. Understanding the breakdown phenomenon caused by the electromagnetic eld leakage is great signi cance for the design of anti-electromagnetic radiation and the design of pulsed electric eld insulation.