Abstract: In order to solve the electromagnetic interference among multi-loaded lines in series when the Tesla-PFN(pulse forming network) long pulse power generator operates under high voltage, the structure of the annular loaded lines in series is optimized and designed, with emphases on the impedance matching between loaded lines and pulse forming line(PFL) and the high voltage insulation of loaded lines. The structure of transition part is optimized, which realizes good impedance matching between loaded lines and PFL and makes the top of the output pulse flat. The electric field distribution of the PFNs is calculated by using a static electric field analysis software, and the distribution is evened by adjusting the shape and location of shielding loop.