  In this paper, the magnetic and transport properties were systematically
studied for EuAg$_4$As$_2$ single crystals, crystallizing in a centrosymmetric
trigonal CaCu$_4$P$_2$ type structure. It was confirmed that two magnetic
transitions occur at $\textit{T}$$_{N1}$ = 10 K and $\textit{T}$$_{N2}$ = 15 K,
respectively. With the increasing field, the two transitions are noticeably
driven to lower temperature. At low temperatures, applying a magnetic field in
the $\textit{ab}$ plane induces two successive metamagnetic transitions. For
both $\textit{H}$ $\parallel$ $\textit{ab}$ and $\textit{H}$ $\parallel$
$\textit{c}$, EuAg$_4$As$_2$ shows a positive, unexpected large
magnetoresistance (up to 202\%) at low fields below 10 K, and a large negative
magnetoresistance (up to -78\%) at high fields/intermediate temperatures. Such
anomalous field dependence of magnetoresistance may have potential application
in the future magnetic sensors. Finally, the magnetic phase diagrams of
EuAg$_{4}$As$_{2}$ were constructed for both $\textit{H}$ $\parallel$
$\textit{ab}$ and $\textit{H}$ $\parallel$ $\textit{c}$.
