In　this　study,　ultrasound-assisted　hydrogen　peroxide-vitamin　C　and　hydrogen　peroxide-Fe2　+　methods　were　used　for　the　limited　hydrolysis　of　Nannochloropsis　sp.　polysaccharide　NSP-3,　and　two　degraded　polysaccharides,　C-NSP-3　and　Fe-NSP-3,　were　obtained,　and　the　degradation　effects　of　the　two　methods　were　compared　by　analysing　the　physicochemical　properties　of　the　polysaccharides　before　and　after　degradation,　as　well　as　their　antioxidant　activities.　The　protein　content,　sulphate　content,　viscosity,　weight　average　molecular　weight,　and　average　particle　size　of　the　polysaccharides　of　Nannochloropsis　sp.　were　significantly　reduced　after　hydrolysis　(P　2　+　was　stronger,　the　degradation　was　better,　and　the　weight　average　molecular　weight　and　average　particle　size　of　the　resulting　polysaccharides　were　smaller.　Infrared　spectroscopy　showed　that　both　hydrolysis　methods　did　not　cause　significant　damage　to　the　polysaccharide　structure,　and　there　was　no　significant　change　in　the　monosaccharide　components　and　major　functional　groups,　which　only　led　to　a　decrease　in　the　content　of　functional　groups.　The　antioxidant　activity　analysis　showed　that　the　polysaccharides　of　Nannochloropsis　sp.　had　certain　antioxidant　capacity,　and　the　limited　hydrolysis　could　significantly　enhance　the　antioxidant　activity　of　the　polysaccharides,　and　their　antioxidant　activities　were　in　the　order　of　C-NSP-3　＞　Fe-NSP-3　＞　NSP-3.　Although　the　molecular　weight　of　Fe-NSP-3　was　lower　than　that　of　C-NSP-3,　the　antioxidant　activity　was　not　as　good　as　that　of　C-NSP-3,　which　may　be　due　to　the　fact　that　the　Fe-NSP-3　was　over-degraded.　This　study　provides　a　theoretical　reference　for　the　development　and　utilization　of　polysaccharides　from　Nannochloropsis　sp.　and　the　selection　of　degradation　methods.　©　2025　China　National　Research　Institute　of　Food　and　Fermentation　Industries　Co.　Ltd.　All　rights　reserved.