Semiconductor heterostructures underpin the design and performance of virtually all modern optoelectronic systems. The clear interface, high crystalline quality, and suitable energy band match lay the foundation for high-performance heterojunction optoelectronic devices. Here, a facile solution epitaxial growth method is demonstrated for controllably growing CHNHPbBr single crystal (SC) on the CHNHPbCl SC, which forms a pn heterojunction with a well-defined interface and high single-crystalline quality. The thickness of the epitaxial layer can be finely controlled by adjusting the growth time. Benefiting from excellent built-in electrical potential at pn heterojunction, the heterojunction detector shows an obvious rectification behavior and high performance in photo-imaging and X-ray detection without any power supply, including high responsivity (26.8 mA W), fast response time (8/613 µs), and high sensitivity of 868 µC Gy cm with a record lowest detectable X-ray dose rate of 15.5 nGy s. Furthermore, it exhibits high-fidelity UV photo-imaging capability at zero bias voltage. In addition, the heterojunction detector shows excellent stability by maintaining 99% of the initial responsivity after 120 days without any encapsulation in the atmosphere. This work enables a significant advance in engineering perovskite SC heterojunction for developing novel and functional devices.