Ultrasmall black phosphorus quantum dots (BPQDs) with an average size of 2.1 ± 0.9 nm are synthesized by using a solvothermal method in a N‐methyl‐2‐pyrrolidone solution. Verified by femto‐second laser Z‐scan measurement, BPQDs exhibit excellent nonlinear optical response with a modulation depth of about 36% and a saturable intensity of about 3.3 GW cm−2. By using BPQDs as optical saturable absorber, the ultrashort pulse with a pulse duration of about 1.08 ps centered at a wavelength of 1567.5 nm is generated in mode‐locked fiber laser. These results suggest that BPQDs may be developed as another kind of promising nanomaterial for ultrafast photonics. A solvothermal synthesis is decribed to produce ultrasmall black phosphorus quantum dots (BPQDs) in large scale. BPQDs exhibit excellent nonlinear optical response and are used as optical saturable absorber. The ultrashort pulse with a pulse duration of about 1.08 ps centered at a wavelength of 1567.5 nm is generated by a mode‐locked fiber laser.