  The newly discovered high-temperature superconductivity in La$_3$Ni$_2$O$_7$
under pressure has attracted a great deal of attentions. The essential
ingredient characterizing the electronic properties is the bilayer NiO$_2$
planes coupled by the interlayer bonding of $3d_{z^2}$ orbitals through the
intermediate oxygen-atoms. In the strong coupling limit, the low energy physics
is described by an intralayer antiferromagnetic spin-exchange interaction
$J_{\parallel}$ between $3d_{x^2-y^2}$ orbitals and an interlayer one
$J_{\perp}$ between $3d_{z^2}$ orbitals. Taking into account Hund's rule on
each site and integrating out the $3d_{z^2}$ spin degree of freedom, the system
reduces to a single-orbital bilayer $t$-$J$ model based on the $3d_{x^2-y^2}$
orbital. By employing the slave-boson approach, the self-consistent equations
for the bonding and pairing order parameters are solved. Near the physically
relevant $\frac{1}{4}$-filling regime (doping $\delta=0.3\sim 0.5$), the
interlayer coupling $J_{\perp}$ tunes the conventional single-layer $d$-wave
superconducting state to the $s$-wave one. A strong $J_{\perp}$ could enhance
the inter-layer superconducting order, leading to a dramatically increased
$T_c$. Interestingly, there could exist a finite regime in which an $s+id$
state emerges.
