The parallel-plate Blumlein line generates pulsed square waveform and is widely used in the field of pulse power technology. This paper finds that the electromagnetic field leakage of planar transmission line will cause the electric field 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 field 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 field leakage has been confirmed by statistics of experimental data. In view of this electric field enhancement effect, this paper proposes an electrode structure design that can reduce the electric field enhancement effect, and the simulation result shows that the optimized electrode structure can significantly reduce the electric field enhancement induced by the electromagnetic field leakage of planar transmission line. Understanding the breakdown phenomenon caused by the electromagnetic field leakage is of great significance for the design of anti-electromagnetic radiation and the design of pulsed electric field insulation.