Searching two-dimensional (2D) half-metallic crystals that are feasible in experiment is essential to develop next-generation nanospintronic devices. Here, a 2D exfoliated MnPSe3 nanosheet with novel magnetism is first proposed based on first-principles calculations. In particular, the evaluated low cleavage energy and high in-plane stiffness indicate that the free-standing MnPSe3 nanosheet can be exfoliated from its bulk structure in experiment. The MnPSe3 nanosheet is an antiferromagnetic semiconductor at its ground state, whereas both electron and hole doping induce its transition from antiferromagnetic semiconductor to ferromagnetic half-metal. Moreover, the spin-polarization directions of 2D half-metallic MnPSe3 are opposite for electron and hole doping, which can be controlled by applying an external voltage gate. The Monte Carlo simulation based on the Ising model suggests the Curie temperature of the doped 2D MnPSe3 crystal is up to 206 K. These advantages render the 2D MnPSe3 crystal with great potentials for application in electric-field controlled spintronic devices.