  In this paper we study the decay estimates of the fourth order
Schr\"{o}dinger operator $H=\Delta^{2}+V(x)$ on $\mathbb{R}^2$ with a bounded
decaying potential $V(x)$. We first deduce the asymptotic expansions of
resolvent of $H$ near the zero threshold in the presence of resonances or
eigenvalue, and then use them to establish the $L^1-L^\infty$ decay estimates
of $e^{-itH}$generated by the fourth order Schr\"{o}dinger operator $H$. Our
methods used in the decay estimates depend on Littlewood-Paley decomposition
and oscillatory integral theory. Moreover, we classify these zero resonances as
the distributional solutions of $H\phi=0$ in suitable weighted spaces. Due to
the degeneracy of $\Delta^{2}$ at zero threshold and the lower even dimension
(i.e. $n=2$), we remark that the asymptotic expansions of resolvent
$R_V(\lambda^4)$ and the classifications of resonances are more involved than
Schr\"odinger operator $-\Delta+V$ in dimension two.
