Abstract: Dielectric wall accelerator is a new type of linear accelerator, which has the advantages of high acceleration gradient, small size and low construction and maintenance cost. The development of high current dielectric wall accelerators requires high voltage ratio pulse forming lines with long pulse output capability. In this paper, a solid-state pulse forming line （Blumlein line） with unmatched load was designed. The open circuit design was used at the load end, and the voltage doubling ratio is higher than that of the pulse forming line with load matching design. At the same time, in order to reduce the size of the pulse forming lines and realize the compactness of the accelerator, the electrode structure in the pulse forming line was adopted S-fold design. Firstly, the electromagnetic simulation program was used to study the influence of several parameters on the output pulse width of Blumlein lines with unmatched load. The parameter design of long pulse width pulse forming line was confirmed by simulation, and four kinds of the pulse forming lines were developed for experimental verification. Secondly, by combining simulation and experiment, the main reasons of the output performance decline of the laminated Blumlein lines were analyzed and the solution was put forward in the paper. Finally, the output performance of multistage Blumlein lines was simulated to confirm the effectiveness of the proposed method. The results show that the coupling effect of the thin plate forming lines in the unmatched load working state is significant. This coupling effect reduces the amplitude of the output edge of the laminated Blumlein lines, which adversely affects the output waveform quality and is the main reason for the degradation of output performance of the laminated Blumlein lines. Increasing the distance between Blumlein lines can reduce the coupling effect, but will increase the length of the acceleration cavity, which is not conducive to the realization of high acceleration gradient. The effect of electric field between Blumlein lines can be effectively reduced by the method of stacking in opposite direction, the coupling between Blumlein lines can be reduced, and the output waveform quality can be improved. The 20-stage stacked Blumlein lines can output long pulses with a peak value of 33 V and pulse width of 154 ns at 1 V input. The average voltage doubling ratio is greater than 1.6, and the plate top with a fluctuation of ±3% accounts for 37.3%. The simulation results of multi-stage Blumlein lines show that the designed S-type plate Blumlein line with unmatched load has better output performance under multi-stage superposition, and can meet the requirements of high current dielectric wall accelerators.