Black phosphorus quantum dots (BPQDs), a type of nanoscale black phosphorus (BP), have fantastic application prospects in various fields. However, the premise of the application of BPQDs depends on its effective preparation. At present, most of preparation processes of BPQDs involve in organic solvents which may be harmful to humans and the environment. Furthermore, some chemical impurities may inevitably be introduced into the final product. In addition, all the preparation processes need to be carried out under an inert gas due to the instability of BPQDs, which makes the reaction conditions more harsh and complicated. Therefore, an efficient and simple method for the preparation of BPQDs by gas exfoliation with the assistance of liquid N₂ (l-N₂) was developed for the first time in this study. This method is environmentally friendly and impurity-free because l-N₂ is a nontoxic liquid that can be gasified to form N₂. The obtained BPQDs were characterized by XRD, Raman, SEM, TEM and UV-Vis techniques and they had a lateral size of 9±3 nm.