Algal　toxins　are　important　metabolites　of　toxic　harmful　algal　blooms　(HABs),　and　their　qualitative　and　qualitative　detection　can　serve　as　early　warning　indicators　for　toxic　HABs,　complementing　traditional　HAB　monitoring　and　improving　the　accuracy　of　early　warning.　Therefore,　this　work　took　the　detection　of　domoic　acid　(DA)　as　an　example　and　prepared　zeolitic　imidazolate　framework-8　(ZIF-8)　with　high　enrichment　performance　and　high　water　stability　and　its　core-shell　composite　material　SiO2@ZIF-8　as　an　adsorbent　filler.　Density　functional　theory　(DFT)　calculations　and　interference　experiments　verified　that　Zn2+　on　SiO2@ZIF-8　played　a　crucial　role　in　enriching　DA　on　SiO2@ZIF-8.　By　using　it　as　a　solid-phase　extraction　(SPE)　filler,　it　showed　excellent　performance　compared　with　other　SPE　columns　(C18/HLB/SAX/ZIF-8).　Therefore,　the　SiO2@ZIF-8　column　was　coupled　to　high-performance　liquid　chromatography-mass　spectrometry　(SPE-HPLC-MS/MS)　to　establish　a　highly　sensitive　detection　method　for　algal　toxins　in　seawater,　which　had　a　wide　linear　range　(12.0-5000.0　ng　L−1),　good　reproducibility　(RSD)　and　low　limit　of　detection　(4.0　ng　L−1),　and　realized　the　monitoring　of　trace　DA　in　the　Pingtan　sea　area　of　Fujian　Province　from　2021　to　2022.　By　comparing　other　HAB　early　warning　indicators　such　as　salinity　and　pH　and　combining　them　with　the　information　released　by　the　Fujian　Provincial　Ocean　and　Fisheries　Bureau,　the　content　of　DA　in　seawater　measured　by　the　established　SPE-HPLC-MS/MS　method　can　provide　reference　information　for　HAB　monitoring　and　early　warning.　©　2023　The　Royal　Society　of　Chemistry