Abstract: Kazal-type serines proteinase inhibitor (KSPI) is very important in the immune reaction of biological body, but the study of KSPI in Hyriopsis cumingii has been rarely reported. In this study, the full length of cDNA sequence of Kazal-type serines proteinase inhibitor(KSPI) gene (accession number: KT901291) of H.cumingii was cloned. It was 1029 bp in length, containing 5′and 3′-UTRs parts of 61 bp and 206 bp, and an ORF of 762 bp encoding 253 amino acids, and the molecular weight was 27397.5 u. The amino acid sequence contains five Kazal domain structures, and the result of homology analysis showed that it has a few of similarity with other species, suggesting that this gene belongs to the typical KSPI gene. Real-time quantitative PCR showed that KSPI gene expressed in a wide range of tissues including the adductor, foot, liver, blood, mantle, gill and gonad, and the highest expression was in mantle, on the contrary, the lowest in liver. After being infected with Aeromonas hydrophila， the expression of KSPI gene had significant increasing in 7 tissues compared with the control group. The results suggested that KSPI gene is very important in the immune reaction of Hyriopsis cumingii.