  Based on density functional theory (DFT), we investigate the electronic
properties of bulk and single-layer ZrTe$_4$Se. The band structure of bulk
ZrTe$_4$Se can produce a semimetal-to-topological insulator (TI) phase
transition under uniaxial strain. The maximum global band gap is 0.189 eV at
the 7\% tensile strain. Meanwhile, the Z$_2$ invariants (0; 110) demonstrate
conclusively it is a weak topological insulator (WTI). The two Dirac cones for
the (001) surface further confirm the nontrivial topological nature. The
single-layer ZrTe$_4$Se is a quantum spin Hall (QSH) insulator with a band gap
86.4 meV and Z$_2$=1, the nontrivial metallic edge states further confirm the
nontrivial topological nature. The maximum global band gap is 0.211 eV at the
tensile strain 8\%. When the compressive strain is more than 1\%, the band
structure of single-layer ZrTe$_4$Se produces a TI-to-semimetal transition.
These theoretical analysis may provide a method for searching large band gap
TIs and platform for topological nanoelectronic device applications.
