A four-stage high-voltage transmission line pulse transformer with a voltage gain of approximately 4 is developed for transforming a high-voltage quasi-rectangular pulse with a pulse duration of several hundred nanoseconds. A method for enhancing the inductive isolation of secondary lines is employed to suppress the pulse droop through winding the coaxial cables onto the magnetic rings. An optimization of the distribution of inductive isolation (i.e., the magnetic rings) is explored for saving the magnetic rings, thus reducing the volume and weight of the transformer. The experimental results indicate that this transformer is capable of transforming the pulse voltage from 25 kV into about 100 kV, simultaneously keeping the pulse waveform almost unchanged. In the circuit simulation with PSpice for this transformer, we obtain a quasi-rectangular pulse of 100 kV when inputting a 25-kV pulse, which is in good agreement with that of the experiment. The result of frequency response indicates that this transformer has the potential to be normally applied in a broad bandwidth of 1 kHz-10 MHz.