Abstract: The spiral line was analyzed through a simple theory, the code simulation and the experimental test. The pulse width of spiral forming line was widened comparing with that of non-spiral forming line, which could be determined by a simple formula and be testified by the results of the code simulation. According to the theory and the code simulation, a water-dielectric Blumlein type of PFL with spiral line was built up and the insulated underlay of spiral line was designed carefully to improve the charging voltage. The experiments were carried out using diode load. As a typical result, a pulse output with a diode voltage of 500 kV and a diode current of 28 kA with an FWHM (full width of half maximum) of 128 ns was obtained when the charging voltage of PFL was up to 550 kV.