Heterostructures, which can possess advantages of materials with different properties, have attracted enormous attention in various research fields including solar cells, photocatalysts, and optical electronic devices. In this work, a 2D/3D atomic epitaxial heterostructure with ultrathin BiOCl nanosheets and YF3:Yb, Tm octahedral crystals was fabricated via the halogen atom exchange method in the solution phase. The epitaxial heterointerface can facilitate energy transfer between BiOCl and YF3:Yb, Tm and suppress the energy quenching induced by grain boundaries. By carrying out single-particle confocal characterization, the energy upconverted by YF3:Yb, Tm is quantitatively confirmed to be transferred to ultrathin BiOCl nanosheets. As a result, YF3:Yb, Tm@BiOCl displays outstanding NIR-driven water splitting and waste-water cleaning properties. This study paves the way to fabricate 2D/3D epitaxial heterostructures, which helps to broaden the application of typical 2D materials.