An　easy-prepared　adsorbent　with　high　stable　and　good　dispersibility　is　especially　valuable　for　the　development　of　magnetic　solid-phase　extraction　(MSPE)　techniques.　In　this　study,　magnetic　nitrogen-doped　carbon　nanotube　cages　(N-CNTCs)　were　synthesized　via　direct　carbonization　of　cobalt　(II)-containing　metal-organic　frameworks.　During　carbonization,　cobalt　ions　inside　the　MOFs　were　converted　into　magnetic　functional　nanoparticles　of　N-CNTCs.　Simultaneously,　large　amounts　of　nitrogen,　originating　from　the　organic　ligands,　were　doped　into　the　carbon　framework.　This　unique　structure　gave　the　N-CNTCs　excellent　chemical　stability,　high　affinity,　and　good　dispersibility.　The　synthesized　magnetic　N-CNTCs　were　then　used　for　MSPE　of　okadaic　acid　(OA)　from　aquatic　samples.　A　simple,　efficient,　and　sensitive　method　for　detecting　and　quantitating　OA　was　developed　by　combining　the　above　sample　pretreatment　technique　with　high-performance　liquid　chromatography　-tandem　mass　spectrometry　(HPLC-MS/MS).　The　resulting　method　boasts　a　linear　dynamic　range　of　3.0–1000.0　pg　mL−1　with　good　linearity　(R2　≥　0.9994).　The　limit　of　detection　(LOD)　and　limit　of　quantification　(LOQ)　were　1.3　pg　mL−1　and　3.0　pg　mL−1,　respectively.　Several　shellfish　and　seafood　samples　were　analyzed　using　the　developed　method,　showing　satisfactory　recoveries　(82.0–107.0%)　and　relative　standard　deviations　(<4.5%).　The　developed　method　was　also　used　to　investigate　the　OA　distribution　in　crab　tissues.　Our　results　demonstrate　that　magnetic　N-CNTCs　are　promising　adsorbents　for　providing　reliable　support　for　the　early　warning　and　tracing　to　the　source　of　algae　toxins.　©　2019　Elsevier　B.V.