  In-plane anomalous Hall effect (IPAHE) is an unconventional anomalous Hall
effect (AHE) with the Hall current flows in the plane spanned by the
magnetization or magnetic field and the electric field. Here,we predict a
stable two-dimensional ferromagnetic monolayer
$\mathrm{Mn}_{3}\mathrm{Si}_{2}\mathrm{Te}_{6}$ with collinear ordering of Mn
moments in the basal plane. Moreover, we reveal that the monolayer
$\mathrm{Mn}_{3}\mathrm{Si}_{2}\mathrm{Te}_{6}$ possesses a substantial
periodic IPAHE due to the threefold rotational symmetry, which can be switched
by changing the magnetization orientation by external magnetic fields. In
addition, we briefly discuss the impacts of moderate strains on the electronic
states and AHE, which lead to a near quantized Hall conductivity. Our work
provides a potential platform for realizing a sizable and controllable IPAHE
that greatly facilatates the application of energy-efficient spintronic
devices.
