The basic principle and the experimental results of measuring energy spectra of high energy pulsed X-ray are reported. The Monte-Carlo code is developed to calculate the energy deposition of high energy photons in each sensitive cell of the energy spectrometer. The intensity of high energy pulsed X-ray created on the Qiangguang-Ⅰ accelerator is measured by using the energy spectrometer. According to the deposited energy and the integral data, the time-resolved high pulsed x-ray energy spectrum is obtained. By using the voltage waveform and the current waveform of the diode, the energy spectrum is calculated in details. The results are in agree with the measured values from the experimental energy spectrum.