The ground level enhancement (GLE) event energy spectrum provides important information about the acceleration and propagation of cosmic ray. In this paper, we analyze the proton flux peak energy spectrum of recent GLE event (2017.09 GLE72) by using GOES15 satellite and neutron monitor experiment data. The method of adjacent averaging smoothing and weighted average are applied to study GOES15 satellite data, and obtain the flux peak and flux peak time. By fitting, the energy spectrum index of proton flux peak is 1.88 in the satellite observation energy range. Again, the energy spectrum index of the neutron monitor observation energy range, 4.86, is obtained by using the new neutron monitor yield function. It can be seen that the peak energy spectrum index given by satellites in the lower energy range (5–433 MeV) is much smaller than that given by the neutron monitor in the higher energy range (0.44–19 GeV). This means that, the energy spectrum in the lower energy range is harder than that in the higher energy range. Hence, the results of the energy spectrum could be explained qualitatively by the re-acceleration mechanism of high energy solar particles. In the low corona region, first, the particles released by the solar flare are accelerated, and the energy spectrum index of the high-energy range is twice that of the low-energy range. Then part of the solar high energy particles from the low corona enter into the CME, where they will be re-acceleration by the shock wave. The GLE72 event high energy range energy spectrum index given by the neutron monitor experiment is 4.86, so the energy spectrum index in low energy range should be 4.86/2 = 2.43. However, the low energy range energy spectrum index is 1.88 (lower than 2.43 in low energy range). The reason may be that the energy spectrum index is further reduced due to the re-acceleration effect in the shock wave generated by the CME. The observation of GLE event is one of the main research subjects of the Large High Altitude Air Shower Observatory (LHAASO). Also, the GLE72 proton peak energy spectrum results provide important information to observe solar high energy particles in the LHAASO experiment.