Abstract: Sub-nanosecond high energy X-ray source is widely used in calibration of time response characteristic and sensitivity of radiation detectors. A compact generator for sub-nanosecond X-ray pulse was developed, covering a floor area of 1.2 m×40 cm with weight of 142 kg. A coaxial pulsed transformer was designed using double conical windings and triple resonance transformer technology. A nanosecond pulse forming line and a self-break oil switch were employed, generating a high voltage pulse with peak value of 370 kV and pulse width of 3.8 ns. The voltage was then increased to 520 kV by multi-stage over-matched transmission lines, and the pulse width was sharpened to 0.5 ns via a peaking-chopping oil switch. The power of the pulse loaded across the AK gap of the X-ray tube was 1.8 GW. The X-ray tube, using a foil array cathode, was designed and analyzed on the basis of Child-Langmuir theory. Radiation with a peak energy flux rate of about 1×10¹⁶ MeV·cm^-2·s^-1 was generated at a distance of 20 cm, and the integral dose was 4.1 mR per pulse.