The misuse of antibiotics drives clinical antimicrobial resistance, with detrimental impacts for human health. Therefore, some new antibacterial materials should be developed to address the overuse of clinical antimicrobials. As a new nanomaterial with good biocompatibility and low toxicity, carbon quantum dots (CQDs) are excellent candidates as novel antimicrobial agents. In this study, we prepared blue-emitting phosphorus-doped carbon quantum dots (P-CQDs) with an optimal excitation wavelength of 370 nm and an optimal emission wavelength of 445 nm by a simple and environmentally-friendly one-step hydrothermal method using keratin as the carbon source. We found that P-CQDs exhibited inhibitory effects on microorganisms, with the minimal inhibitory concentration (MIC) values of 0.19 mg/mL against Gram-positive bacteria (S. aureus) and 0.31 mg/mL against Gram-negative bacteria (E. coli). The experimental results implied these stable fluorescence properties of P-CQDs possess good antibacterial activity. Thus, these findings indicate the P-CQDs based on biosynthesized keratin could potentially be used as an excellent antibacterial agent.