  In the paper, we study the properties of the $Z$-boson hadronic decay width
by using the $\mathcal{O}(\alpha_s^4)$-order quantum chromodynamics (QCD)
corrections with the help of the principle of maximum conformality (PMC). By
using the PMC single-scale approach, we obtain an accurate renormalization
scale-and-scheme independent perturbative QCD (pQCD) correction for the
$Z$-boson hadronic decay width, which is independent to any choice of
renormalization scale. After applying the PMC, a more convergent pQCD series
has been obtained; and the contributions from the unknown
$\mathcal{O}(\alpha_s^5)$-order terms are highly suppressed, e.g.
conservatively, we have $\Delta \Gamma_{\rm Z}^{\rm had}|^{{\cal
O}(\alpha_s^5)}_{\rm PMC}\simeq \pm 0.004$ MeV. In combination with the known
electro-weak (EW) corrections, QED corrections, EW-QCD mixed corrections, and
QED-QCD mixed corrections, our final prediction of the hadronic $Z$ decay width
is $\Gamma_{\rm Z}^{\rm had}=1744.439^{+1.390}_{-1.433}$ MeV, which agrees with
the PDG global fit of experimental measurements, $1744.4\pm 2.0$ MeV.
