Alginate　oligosaccharide　(AOS)　was　enzymatically　prepared　and　had　a　degree　of　polymerization　(DP)　of　2-6.　AOS　exhibited　evident　antioxidant　efficacy　in　ABTS+　scavenging　activity,　hydroxyl　radical　scavenging　capacity,　and　total　reducing　power.　The　freshness　of　the　prawns,　which　were　treated　with　AOS　and　e-polylysine　(e-PL),　was　evaluated　by　measuring　indicators　such　as　total　viable　bacteria　(TVC),　pH,　total　volatile　basic　nitrogen　(TVB-N),　sensory,　texture,　and　melanosis　during　storage　period.　The　texture　properties　and　sensory　scores　of　prawns　treated　with　AOS　and　e-PL　were　significantly　improved　(P　＜　0.05)　in　comparison　with　the　control　group.　Meanwhile,　the　increase　in　TVC,　pH,　TVB-N,　and　melanosis　scores　of　prawns　treated　with　AOS　and　e-PL　were　effectively　delayed　(P　＜　0.05)　in　comparison　with　control　group.　Moreover,　the　quality　of　prawns　treated　with　8　g/L　AOS　was　better　than　that　of　prawns　treated　with　6　g/L.　AOS　could　prolong　the　shelf　life　of　prawns　by　2　days　at　least.　Therefore,　AOS　is　a　natural　biological　preservative　in　providing　an　effective　and　safe　way　to　preserve　freshness　of　prawns.