Black phosphorus (BP) nanosheets can quench the fluorescence of a fluorescent dye, and based on this property, the deoxyribonucleic acid(DNA) aptamer which can specifically recognize the human breast cancer cells MCF7 are synthesized and labeled with 5-carboxyfluorescein (FAM). When the DNA aptamer is mixed with BP nanosheets, its fluorescence is quenched. However, in the presence of MCF7 cells, the DNA aptamer can recognize the MCF7 cells and interact with these cells, and the disassociation of DNA aptamer from BP nanosheets occurs and then the restoration of fluorescence is realized. The fluorescence intensity shows the linear relationship with the number of cancer cells. The experimental results indicate that it is highly efficient, specific and sensitive to use BP nanosheets for detecting breast cancer cells and this method can be applied in the detection of other kinds of cancer cells.