Few-layer two-dimensional (2D) semiconductor nanosheets with a layer-dependent band gap are attractive buildingblocks for large-area thin-film electronics. A general approach is developed to fast prepare uniform and phase-pure 2H-WSe2 semiconducting nanosheets at a large scale, which involves the supercritical carbon dioxide (SC-CO2) treatment and a mild sonication-assisted exfoliation process in aqueous solution. The as-prepared 2H-WSe2 nanosheets preserve the intrinsic physical properties and intact crystal structures, as confirmed by Raman, x-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscope (STEM). The uniform 2H-WSe2 nanosheets can disperse well in water for over six months. Such good dispersivity and uniformity enable these nanosheets to self-assembly into thickness-controlled thin films for scalable fabrication of large-area arrays of thin-film electronics. The electronic transport and photoelectronic properties of the field-effect transistor based on the self-assembly 2H-WSe2 thin film have also been explored.