Tuning the topological and magnetic properties of materials by applying an electric field is widely used in spintronics. In this work, we find a topological phase transition from topologically trivial to nontrivial states at an external electric field of about 0.1 V/Å in a MnBi2Te4 monolayer that is a topologically trivial ferromagnetic semiconductor. It is shown that when electric field increases from 0 to 0.15 V/Å, the magnetic anisotropy energy (MAE) increases from about 0.1 to 6.3 meV, and the Curie temperature TC increases from 13 to about 61 K. The increased MAE mainly comes from the enhanced spin-orbit coupling due to the applied electric field. The enhanced TC can be understood from the enhanced p−d hybridization and decreased energy difference between p orbitals of Te atoms and d orbitals of Mn atoms. Moreover, we propose two Janus materials, MnBi2Se2Te2 and MnBi2S2Te2 monolayers with different internal electric polarizations, which can realize the quantum anomalous Hall effect (QAHE) with Chern numbers C=1 and C=2, respectively. Our study not only exposes the electric field induced exotic properties of the MnBi2Te4 monolayer but also proposes materials to realize QAHE in ferromagnetic Janus semiconductors with electric polarization.