Based on over-damped RLC nonsynchronous discharge circuits, a novel high voltage rectangular waveform forming method was proposed. The theoretical analysis showed that the double exponential waveform approaches the rising edge and the flattop of rectangular waveform in a particular situation. Meanwhile the numerical simulation showed the tail of double exponential waveform can be cut off by the artificial current zero. A prototype was designed and tested. The results showed that the prototype can output an adjustable unipolar rectangular pulse with 17 kV amplitude, 330 ns ~ 5.8 μs flattop duration and 110 ~ 350 ns rising time on insulation specimen. The rising time, the positive flattop duration and the negative one of rectangular pulse are adjustable independently. The waveform is not sensitive to specimen variety.