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<i>Aeromonas salmonicida</i>, a psychrophilic bacterial pathogen, is widely distributed in marine freshwater, causing serious economic losses to major salmon farming areas in the world. At present, it is still one of the most important pathogens threatening salmon farming. Hcp (haemolysin-coregulated protein) is an effector protein in the type-VI secretion system (T6SS), which is secreted by T6SS and functions as its structural component. The results of our previous genomic sequencing showed that <i>hcp</i> existed in the mesophilic <i>A. salmonicida</i> SRW-OG1 isolated from naturally infected <i>Epinephelus coioides</i>. To further explore the role of Hcp in <i>A. salmonicida</i> SRW-OG1, we constructed an <i>hcp</i>-RNAi strain and verified its effect on the virulence of <i>A. salmonicida</i>. The results showed that compared with the wild strain, the <i>hcp</i>-RNAi strain suffered from different degrees of decreased adhesion, growth, biofilm formation, extracellular product secretion, and virulence. It was suggested that <i>hcp</i> may be an important virulence gene of <i>A. salmonicida</i> SRW-OG1.