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.