  We investigate topological properties in the Fe(Te,Se) thin films. We find
that the single layer FeTe$_{1-x}$Se$_x$ has nontrivial $Z_2$ topological
invariance which originates from the parity exchange at $\Gamma$ point of
Brillouin zone. The nontrivial topology is mainly controlled by the Te(Se)
height. Adjusting the height, which can be realized as function of $x$ in
FeTe$_{1-x}$Se$_x$, can drive a topological phase transition. In a bulk
material, the two dimensional $Z_2$ topology invariance is extended to a strong
three-dimensional one. In a thin film, we predict that the topological
invariance oscillates with the number of layers. The results can also be
applied to iron-pnictides. Our research establishes FeTe$_{1-x}$Se$_x$ as a
unique system to integrate high T$_c$ superconductivity and topological
properties in a single electronic structure.
