  Prompt extra power-law (PL) spectral component is discovered in some bright
gamma-ray bursts (GRBs), which usually dominates the spectral energy
distribution below tens of keV or above about 10 MeV. However, its origin is
still unclear. In this paper, we present a systematic analysis 13 Fermi short
GRBs as of August 2020, with the contemporaneous keV-MeV and GeV detections at
the prompt emission phase. We find that the extra PL component is a ubiquitous
spectral feature for short GRBs, showing up in all 13 analyzed GRBs. The PL
indices are mostly harder than -2.0, which may be well reproduced by
considering the electromagnetic cascade induced by ultra-relativistic protons
or electrons accelerated in the prompt emission phase. The average flux of
these extra PL components positively correlates with that of the main spectral
components, which implies they may share the same physical origin.
