High-voltage pulse modulator has broad applications in industry. In order to pursue the qualities of compactness, solidification, and long life time, a high-voltage pulse modulator based on a helical Blumlein pulse forming line (HBPFL),a Marx generator and a self-coupling saturable pulse transformer (SPT) with fully cylindrical coaxial conductors is put forward and investigated in this paper. A new method that the fully cylindrical SPT simultaneously works as the charging pulse transformer and magnetic switch of the HBPFL is put forward and demonstrated. Traditional spark gap is substituted by the SPT to enable the features of solidification, compactness, and long life time of the modulator. Experimental results showed that the SPT had good response characteristics to short sinusoidal pulse and 100 ns-range square pulses. The fully cylindrical SPT driven by the 50–70 kV Marx generator can suppress the saturated inductance of the secondary windings to a level less than 500 nH, due to the strong reversed mutual induction between cylindrical windings after the core saturated. It also demonstrated that the pulse modulator was able to deliver a high-voltage pulse to a 160 Ω load, with amplitude of 148 kV, pulse duration of 130 ns, and pulse rise time ranging from 60 to 105 ns.