  By means of first-principles calculations and crystal structure searching
techniques, we predict that a new NaCaBi family crystallized into the
ZrBeSi-type structure (\ie $P6_{3}/mmc$) are strong topological insulators
(STIs). Taking $P6_{3}/mmc$ NaCaBi as an example, the calculated band structure
indicates that there is a band inversion between two opposite-parity bands at
the $\Gamma$ point. In contrast to the well-known Bi$_2$Se$_3$ family, the band
inversion in the NaCaBi family has already occured even without spin-orbit
coupling (SOC), giving rise to a nodal ring surrounding $\Gamma$ in the $k_z=0$
plane (protected by $M_z$ symmetry). With time reversal symmetry $(\cal T)$ and
inversion symmetry $(\cal I)$, the spinless nodal-line metallic phase protected
by $[{\cal TI}]^2=1$ is the weak-SOC limit of the spinful topological
insulating phase. Upon including SOC, the nodal ring is gapped, driving the
system into a STI. Besides inversion symmetry, the nontrivial topology of
NaCaBi can also be indicated by $\bar{6}$ symmetry. More surprisingly, the
SOC-induced band gap in NaCaBi is about 0.34 eV, which is larger than the
energy scale of room temperature. Four other compounds (KBaBi, KSrBi, RbBaBi
and RbSrBi) in the family are stable at ambient pressure, both in
thermodynamics and lattice dynamics, even though the gaps of them are smaller
than that of NaCaBi. Thus, they provide good platforms to study topological
states both in theory and experiments.
