This paper introduces The fractional ratio pulse modulator for S-band 50 MW klystron, which has the characteristics of pulse transformer ratio of 1∶300, primary voltage of 1.25 kV and output peak power of 113 MW. The selection of five sub cores, the design of transformation ratio, the calculation of primary voltage and current, and the calculation of average output power are carried out. It is analyzed that the total inductance of single discharge circuit which affects the output front of solid-state modulator should be less than 0.4 μH. The loop inductance is distributed in the discharge module, the discharge cable, the internal wiring of the pulse transformer and the primary leakage inductance. The low inductance design of the internal loop of the discharge module, the low leakage inductance design of the pulse transformer and the modeling and simulation results are mainly analyzed. The results show that the high-power all solid-state pulse modulator with 312 kV output pulse voltage, 360 A pulse current and less than 1.4 μs front and back edges can be realized. It has promising applications in the fields of accelerator driving RF power source, high-power radar transmitter and so on, which need high-voltage pulse modulator.