Lead halide perovskite nanocrystals and their heterostructures have achieved substantial advances in optoelectronics; however, their inherent material instability and lead toxicity have driven research on alternative material systems. Herein, solution-processable heterostructures composed of lead-free double perovskite Cs2AgBiBr6 nanocrystals and BiOCl nanosheets were prepared through a colloidal synthesis method. Defect states were present in BiOCl and benefited carrier generation, recombination and transport in Cs2AgBiBr6. As a result, the light emission of the Cs2AgBiBr6 nanocrystals was greatly enhanced at low temperatures, and the photodetector based on the Cs2AgBiBr6/BiOCl heterostructure exhibited a much improved on-off ratio compared to the device based on Cs2AgBiBr6 alone. Our work highlights the complex nature and impact of two-dimensional heterostructure assembly on the optoelectronic properties of lead-free double perovskites and demonstrates their great potential toward environmentally friendly optoelectronic devices.