The invention discloses a biodegradable black phosphorus-based radiotherapy sensitizer and a preparation method and application thereof. The preparation method of the biodegradable black phosphorus-based radiotherapy sensitizer comprises the following steps of adding a soluble bismuth salt solution into a black phosphorus nanosheet solution, standing for 0.5h or more, and centrifuging, so as to obtain the black phosphorus-based radiotherapy sensitizer, wherein the mass ratio of the bismuth salt to the black phosphorus nanosheet is (0.1 to 1.0):1.0. The prepared black phosphorus-based radiotherapy sensitizer can be used for preparing anti-tumor medicines, and the medicines are jointly used with an X-ray radiotherapy method, so as to reach better anti-tumor function. The biodegradable blackphosphorus-based radiotherapy sensitizer has the advantages that the large-scale preparation of the biodegradable black phosphorus-based X-ray power-therapy photo-sensitizer is realized; the foundation is laid for application in fields of photoelectrical devices, solar batteries, lithium batteries, biomedicines and the like.