The paper introduces the principle of pulse form network (PFN), the PSpice simulation result of Marx-PFN and the design process of 200 kV long pulse power generator. A 200 kV long pulsed power generator based on five-stage Marx-PFN has been built, with network impedance of 14 Ω. The results indicate that the FWHM of the output pulse with 3 μs flat-top is 43 μs,the ripples of the top of the pulse are less than 1%,the risetime of the pulse is less than 500 ns, and the voltage of the pulse is about 200 kV. The effect of earthing resistance and induction on waveform is analyzed too.