  We investigate the relationship between the blue shifts of a hot emission
line and the nonthermal emissions in microwave and hard X-ray (HXR) wavelengths
during the precursor of a solar flare on 2014 October 27. The flare precursor
is identified as a small but well-developed peak in soft X-ray and
extreme-ultraviolet passbands before the GOES flare onset, which is accompanied
by a pronounced burst in microwave 17 & 34 GHz and HXR 25-50 keV. The slit of
Interface Region Imaging Spectrograph (IRIS) stays at one ribbon-like transient
during the flare precursor, where shows visible nonthermal emissions in NoRH
and RHESSI images. The IRIS spectroscopic observations show that the hot line
of Fe XXI 1354.09 A (logT ~ 7.05) displays blue shifts, while the cool line of
Si IV 1402.77 A (logT ~ 4.8) exhibits red shifts. The blue shifts and red
shifts are well correlated to each other, indicative of an explosive
chromospheric evaporation during the flare precursor particularly combining
with a high nonthermal energy flux and a short characteristic timescale. In
addition, the blue shifts of Fe XXI 1354.09 A are well correlated with the
microwave and HXR emissions, implying that the explosive chromospheric
evaporation during the flare precursor is driven by nonthermal electrons.
