  For a newly discovered iron-based high T_c superconducting parent material
KFe2Se2, we present an effective three-dimensional five-orbital tight-binding
model by fitting the band structures. The three t2g-symmetry orbitals of the
five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, in
agreement with recent angle-resolved photoemission spectroscopy experiments. To
understand the groundstate magnetic structure, the two- and three-dimensional
dynamical spin susceptibilities within the random phase approximation are
investigated. It obviously shows a sharp peak at wave vector $\mathbf{Q}$
$\thicksim$ ($\pi$, $\pi$), indicating the magnetic instability of {\it
N$\acute{e}$el}-type antiferromagnetic rather than ($\pi$/2, $\pi$/2)-type
antiferromagnetic ordering. While along $\emph{c}$ axis, it exhibits a
ferromagnetic coupling between the nearest neighboring FeSe layers. The
difference between the present results and the experimental observation in
KxFe2-ySe2 is attributed to the presence of Fe vacancy in the latter.
