According to the experiment, we analyze the propagation process of the electromagnetic wave on the pulse forming line with water-dielectric, which is the long pulse high power microwave-driven source. And then an equivalent method of discharge process is put forward to study the cause of the flattop waveform distortion. We ind the relationship between the variation trend of the output flattop waveform and the voltage waveform on the pulse forming line, which is validated through the Pspice simulation. By adjusting the breakover time of laser triggered switch, we obtain an output pulse with a flattop of 60 ns, FWHM (full width of half maximum) of 150 ns and voltage amplitude of several hundred kV, showing the distinct improvement on the flattop waveform.