In this paper, we show how to carry out a relatively more realistic and complete reconstruction of supernova neutrino spectra in the future large liquid-scintillator detectors, by implementing the method of singular value decomposition with a proper regularization. For a core-collapse supernova at a distance of 10 kpc in the Milky Way, its ${\overline{\nu }}_e$ spectrum can be precisely determined from the inverse beta-decay process ${\overline{\nu }}_e+p\to e^{+}+n$, for which a 20 kiloton liquid-scintillator detector with the resolution similar to the Jiangmen Underground Neutrino Observatory may register more than 5000 events. We have to rely predominantly on the elastic neutrino-electron scattering $\nu +e^{-}\to \nu +e^{-}$ and the elastic neutrino-proton scattering $\nu +p\to \nu +p$ for the spectra of ${\nu }_e$ and ${\nu }_x$, where $\nu$ denotes collectively neutrinos and antineutrinos of all three flavors and ${\nu }_x$ for ${\nu }_{\mu }$ and ${\nu }_{\tau }$ as well as their antiparticles. To demonstrate the validity of our approach, we also attempt to reconstruct the neutrino spectra by using the time-integrated neutrino data from the latest numerical simulations of delayed neutrino-driven supernova explosions.