Objective To construct a recombinant Lactococcus lactis expression system of phage lysine Cp51, and study its antibacterial activity against Clostridium perfringensin type A in vitro.Method Phage lysin Cp51 gene was inserted into the prokaryotic expression vector PNZ8148, and then the recombinant vector PNZ8148-Cp51 was constructed and transformed into L. lactis NZ9000. Nisin was used to induce expression of soluble protein Cp51. The antibacterial activity of the recombinant protein against C. perfringens type A was detected by kinetic turbidimetric assay in vitro. The inhibitory effect of recombinant L. lactis on the proliferation of C. perfringens type A was tested by mixed cultivation.Result The recombinant phage lysin Cp51 was successfully expressed in L. lactis and the Cp51 gene was 1 268 bp in length. The recombinant protein had strong antibacterial activity against C. perfringens type A at the concentration above 1 μg·mL^-1 in vitro. The recombinant L. lactis also had certain inhibitory effect on C. perfringens type A with the number of C. perfringens decreased from 9×10⁹ CFU·mL^-1 to 9×10⁸ CFU·mL^-1 after mixed cultivation.Conclusion Recombinant L. lactis expressing phage lysin Cp51 which against C. perfringens type A has been successfully constructed, which provides a basis for further optimization of expression and clinical application.