  Three-dimensional (3D) Weyl semimetal Mn$_3$Sn has attracted tremendous
attention due to its great application potential. However, the complex magnetic
structures at different temperature intervals make it extremely difficult to
unravel the underlying electronic structures of Mn$_3$Sn. Here, we perform
temperature-dependent optical spectroscopy measurements on single crystalline
Mn$_3$Sn to investigate its charge dynamics. We find that both of the optical
reflectivity $R(\omega)$ and conductivity $\sigma_1(\omega)$ evolve very
smoothly across the magnetic phase transition at $T_M$ = 285 K, where the giant
anomalous Hall effect (AHE) at room temperature drops significantly.
Furthermore, two linearly increasing segments of $\sigma_1(\omega)$ are
observed in the whole temperature range from 300 K to 10 K, indicating that the
existence of Weyl fermions is very robust against the magnetic phase
transition. In addition, the Weyl points closest to the Fermi level $E_F$ are
identified to be located about 101 meV away from $E_F$ at 10 K, and the
associated Fermi velocity is about 2.55 $\times 10^7$ cm/s. Our results reveal
that the phase transition at $T_M$ only generates subtle modification to the
band structure, which helps to further uncover the mechanism of the dramatic
change of AHE in Mn$_3$Sn.
