This paper introduces the basic principle and design ideas of absorptive method for X-ray energy spectrum measurement. Si-PIN detectors array and Cu, Al attenuation materials were used to design the energy spectrometer combining theoretical calculation and experimental study. The collimating and anti-interference system was developed to obtain higher signal-to-noise ratio waveforms. The influence of scattering was researched. The different attenuation waveforms of pulsed hard X-ray at ShanguangⅡ accelerator were measured experimentally in the vacuum environment. The energy spectrum was calculated. The maximum energy of X-ray was about 600 keV, and the average energy was about 89.1 keV, which agreed with the calculation results.