Abstract: Pulse forming network (PFN) is often used in high-power solid-state modulator, microwave driver and laser exciter to obtain high voltage and long pulse output with wide flat-top. Aiming at the common used Rayleigh PFN with equal inductor and capacitor, according to the application requirement of wide flat-top and low ripple, the optimal design technology of PFN is studied, and the optimization algorithm based on simplex optimization method is proposed. The work is mainly carried out in two cases: first, the capacitance is uniform, and the optimal output performance is obtained by optimizing the inductance value; second, the capacitors are constrained, and through optimizing the inductance values under different capacitance arrangement, the optimal capacitance arrangement and the corresponding inductance value are worked out. The optimization results show that the quasi-square-wave pulse output can be obtained in both cases. A new method for design and engineering implementation of quasi-square-wave PFN can be provided. The theoretical calculation and circuit simulation results indicate that the proposed method is reasonable and practicable.