In order to study the functional role of chitinase in the infection of diatoms by Aeromonas salmonidae,Aeromonas salmonicida and its knockout strain were uesd to be co-culture with diatoms and explore the difference of its function.The results showed that:after 48 hours of co-cultivation of Chaetoceros muelleri with wild strains and knockout strains of Aeromonas salmonicida, the wild strains had a killing rate of 9.18% against Chaetoceros muelleri, while the knockout strains had no significant algae killing Activity; by scanning electron microscopy, it was further found that wild strains of bacteria could lyse the cell wall of Chaetoceros muelleri, but the knockout strains could not. The results confirmed that Aeromonas salmonicida has a killing effect on algae due to its chitinase.