  Relativistic effects are important in the rigorous study of heavy quarks. In
this paper, we study the relativistic corrections of semileptonic $B_c$ decays
to charmonium with the instantaneous Bethe-Salpeter method. Within the
Bethe-Salpeter framework, we use two methods to study the relativistic effects.
One of them is to expand the transition amplitude in powers of $\vec{q}$ which
is the relative momentum between the quark and antiquark, and the other is to
expand the amplitude base on the wave functions. In the level of decay width,
the results show that, for the transition of $B_c\to \eta_c$, the relativistic
correction is about $22\%$; for $B_c\to J/\psi$, it is about $19\%$; the
relativistic effects of $1P$ final states are about $14\sim46\%$ larger than
those of $1S$ final states; for $2S$ final states, they are about $19\sim 28\%$
larger than those of $1S$ final states; for $3S$ final states, they are about
$12\sim13\%$ larger than those of $2S$ final states; for $2P$ final states,
they are about $10\sim14\%$ larger than those of $1P$ final states; for $3P$
final states, they are about $7\sim12\%$ larger than those of $2P$ final
states. We conclude that the relativistic corrections of the $B_c$ decays to
the orbitally or radially excited charmonium (2S, 3S, 1P, 2P, 3P) are quite
large.
