The Blumlein pulse forming network (PFN) has widely beenused in pulsed power technology to generate square waves with short pulse widths. In this paper, we developed a repetitive frequency square wave generator based on Blumlein PFN and pseudospark switch (PSS). A Blumlein PFN with unequal capacitances has been proposed, and the PFN parameters have been optimized for better output waveforms. A single-gap PSS with a withstand voltage of 40kV and a repetitive frequency of 100Hz has been designed to switch the Blumlein PFN. The experiment results show that the square wave generator can output pulses with a voltage of 26kV, a rise time of 25ns, and a pulse width of 90ns on a matched load of 11Ω. It has operated steadily for 10h with a repetitive frequency of 100Hz, and the jitter remains at around 1ns after 1.05 × 106 shots.