Research on the characteristics of X-ray emission in repetitive pulsed discharge is associated with the behavior of runaway electrons and the mechanism of nanosecond pulsed discharge. In the experiments, X-ray emission in the repetitive pulsed discharge with a rise time of ～15 ns and an FWHM of 30-40 ns in atmospheric air is directly measured by a detection system consisting of NaI scintillator and a photomultiplier tube, and the energy range of the X-ray detector is demonstrated to be between 10 and 130 keV. Results show that main part of the energy of X-ray is from 20 keV to 90 keV, and a little X-ray with an the energy of less than 20 keV or more than 90 keV is detected. X-ray emission increases with the increase of pulse repetition frequency, and it has a peak value with the variation of air gap spacing. There is a maximum value of X-ray emission when the nanosecond discharge appears in a diffuse mode.