Abstract: In order to realize the miniaturization of the 25 GW level dual-output ultra-wide spectrum HPM pulse generator, a Tesla transformer with open circuit magnetic core as the primary power source is developed, which is fixed in the Blumlein pulse forming line. Theoretical analysis of the Tesla transformer is carried out, and the method to estimate the electric parameters such as inductances of the primary and secondary windings by using a simplified magnetic circuit model is presented. The approach to calculating the sectional area of the magnetic core and a fabricating technique of the magnetic core are also described. The Tesla transformer can generate the maximum output voltage of 880 kV, and its charge time and coupling coefficient are 20 s and 0.95, respectively. The experimental result is in good agreement with the theoretical design.