Novel double pulse forming lines (PFLs) are fabricated with two separate coaxial single PFLs with the same dimensions with each inner cylinder of the PFL negatively charged to improve the breakdown voltage of PFL and the energy storage density of de-ionized water. A spiral line is adopted for the single PFL to raise the impedance of the PFL and prolongs the pulse duration. Ethylene glycol is mixed with de-ionized water to raise the leakage resistance and the impedance of the single PFL. A double structure is adopted to reduce the breakdown voltage requirement of the transformer and the switch. At the same time, the double structure is an advantage to raise the impedance of the PFL.