  We report the physical properties of ThRu$_3$Si$_2$ featured with distorted
Ru kagome lattice. The combined experiments of resistivity, magnetization and
specific heat reveal bulk superconductivity with $T_{\rm{c}}$ = 3.8 K. The
specific heat jump and calculated electron-phonon coupling indicate a moderate
coupled BCS superconductor. In comparison with LaRu$_3$Si$_2$, the calculated
electronic structure in ThRu$_3$Si$_2$ shows an electron-doping effect with
electron filling lifted from 100 meV below flat bands to 300 meV above it. This
explains the lower superconducting transition temperature and weaker electron
correlations observed in ThRu$_3$Si$_2$. Our work suggests the $T_{\rm{c}}$ and
electronic correlations in kagome superconductor could have intimate connection
with the flat bands.
