  This work reports a precise measurement of the reactor antineutrino flux
using 2.2 million inverse beta decay (IBD) events collected with the Daya Bay
near detectors in 1230 days. The dominant uncertainty on the neutron detection
efficiency is reduced by 56% with respect to the previous measurement through a
comprehensive neutron calibration and detailed data and simulation analysis.
The new average IBD yield is determined to be
$(5.91\pm0.09)\times10^{-43}~\rm{cm}^2/\rm{fission}$ with total uncertainty
improved by 29%. The corresponding mean fission fractions from the four main
fission isotopes $^{235}$U, $^{238}$U, $^{239}$Pu, and $^{241}$Pu are 0.564,
0.076, 0.304, and 0.056, respectively. The ratio of measured to predicted
antineutrino yield is found to be $0.952\pm0.014\pm0.023$
($1.001\pm0.015\pm0.027$) for the Huber-Mueller (ILL-Vogel) model, where the
first and second uncertainty are experimental and theoretical model
uncertainty, respectively. This measurement confirms the discrepancy between
the world average of reactor antineutrino flux and the Huber-Mueller model.
