  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 $\sim$10 MeV. However, its origin is
still unclear. In this letter, we present a systematic analysis for 13
\textit{Fermi} short GRBs as of August 2020, with significant 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.
