  The observation of Type Ia supernovae (SNe Ia) plays an essential role in
probing the expansion history of the universe. But the possible presence of
cosmic opacity can degrade the quality of SNe Ia. The gravitational-wave (GW)
standard sirens, produced by the coalescence of double neutron stars and black
hole--neutron star binaries, provide an independent way to measure the
distances of GW sources, which are not affected by cosmic opacity. In this
paper, we first propose that combining the GW observations of third-generation
GW detectors with SN Ia data in similar redshift ranges offers a novel and
model-independent method to constrain cosmic opacity. Through Monte Carlo
simulations, we find that one can constrain the cosmic opacity parameter
$\kappa$ with an accuracy of $\sigma_{\kappa}\sim0.046$ by comparing the
distances from 100 simulated GW events and 1048 current Pantheon SNe Ia. The
uncertainty of $\kappa$ can be further reduced to $\sim0.026$ if 800 GW events
are considered. We also demonstrate that combining 2000 simulated SNe Ia and
1000 simulated GW events could result in much severer constraints on the
transparent universe, for which $\kappa=0.0000\pm0.0044$. Compared to previous
opacity constraints involving distances from other cosmic probes, our method
using GW standard sirens and SN Ia standard candles at least achieves
competitive results.
