  Disorder is important in the study of quantum spin liquids, but its role on
the spin dynamics remains elusive. Here, we explore the disorder effect by
investigating the magnetic-field dependence of the low-energy magnetic
excitations in a triangular-lattice frustrated magnet YbZnGaO$_4$ with
inelastic neutron scattering. With an intermediate field of 2.5 T applied along
the $c$-axis, the broad continuum at zero field becomes more smeared both in
energy and momentum. With a field up to 10 T, which fully polarizes the
magnetic moments, we observe clear spin-wave excitations with a gap of
$\sim$1.4 meV comparable to the bandwidth. However, the spectra are
significantly broadened. The excitation spectra both at zero and high fields
can be reproduced by performing classical Monte Carlo simulations which take
into account the disorder effect arising from the random site mixing of
nonmagnetic Zn$^{2+}$ and Ga$^{3+}$ ions. These results elucidate the critical
role of disorder in broadening the magnetic excitation spectra and mimicking
the spin-liquid features in frustrated quantum magnets.
