  We extract 18 candidate short gamma-ray bursts (SGRBs) with precursors from
660 SGRBs observed by {\em Fermi} and {\em Swift} satellites, and carry out a
comprehensive analysis on their temporal and spectral features. We obtain the
following results: (1) For a large fraction of candidates, the main burst
durations are longer than their precursor durations, comparable to their
quiescent times from the end of precursors to the beginning of their main
bursts. (2) The average flux of precursors tends to increase as their main
bursts brighten. (3) As seen from the distributions of hardness ratio and
spectral fitting, the precursors are slightly spectrally softer with respect to
the main bursts. Moreover, a significant portion of precursors and all main
bursts favor a non-thermal spectrum. (4) The precursors might be a probe of the
progenitor properties of SGRBs such as the magnetic field strength and the
crustal equation of state if they arise from some processes before mergers of
binary compact objects rather than post-merger processes.
