Facile and scalable synthesis of perovskite heterostructures with well-controlled heterointerfaces remains challenging. Herein, we developed a simple one-step solution method to prepare 3D/2D CsPbBr3/PEA2PbBr4 perovskite heterostructures with a well-defined epitaxial structure in the gram scale. The formation mechanism was detailed by using in situ time-resolved photoluminescence (PL) spectroscopy analysis. In addition, a series of 3D/2D epitaxial heterostructures were also prepared by changing the organic cations or halogen anions. Due to the effective charge separation and transfer, photodetectors based on the type-II 3D/2D CsPbBr3/PEA2PbBr4 heterostructures showed up to 120 times higher photoresponsivities and 50 times higher on/off ratios compared to devices based on single component perovskites.