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The gamma-ray burst GRB 180720B is very peculiar. On one hand, someinteresting features have been found by performing the detailed time-resolvedspectral analysis in the prompt phase. First, the `flux-tracking' pattern isexhibited both for the low energy spectral index $\alpha$ and the peak energy$E_{p}$ in the Band function. Second, some parameter relations show strongmonotonous positive correlations, include $E_{p}-F$, $\alpha-F$,$E_{p}-\alpha$, and $E_{p}-L_{\gamma,iso}$ for all time-resolved spectra.Lastly, it should be noted that the values of $\alpha$ do not exceed thesynchrotron limits (from $-\frac{3}{2}$ to $-\frac{2}{3}$). On the other hand,the photons with the energy of $\gtrsim$ 100 MeV were detected by LAT both inthe prompt phase and afterglow. Notably, the 5 GeV photon was observed at 142 safter the GBM trigger. The spectrum of this burst in the LAT range can bedescribed as $F_{\nu}\propto \nu^{-1.3} t^{-1.54\pm0.02}$ in the afterglowphase. And there are six GeV photons during the X-ray flare when the lowerenergy emission is fading to a weaker level. We try to give reasonableinterpretations of the mechanism for prompt emission and the high energyemission (100 MeV to GeV) in the afterglow. The interpretations suggesting thatsynchrotron origin can account for the prompt emission and synchrotronself-Compton radiation can account for both the spectrum and temporal behaviorof the 100 MeV to GeV afterglow emission that have been accepted by us.

Origin for the Prompt Spectral Evolution Characteristics and High Energy Emission during the X-Ray Flare in GRB 180720B