Abstract: A high power water dielectric double-pulse compact modulator has been designed using the folded coaxial transmission line method combined with the spiral line method. With two spiral transmission lines, an air core transformer and two trigger switches, double long pulses could be obtained at each end of the pulse forming line(PFL). The wave process of the pulse modulator was analyzed. The influences of the transition part of the modulator on the load voltage were also discussed. Pspice circuit was used to calculate the voltage and current at the dummy loads, and the charging voltage of the PFL. Experiments were done using high power coaxial line at 20 kV, and the results are in good agreement with the theoretical analysis and simulation results.