Saxitoxin　(STX)　has　high　toxicity,　and　is　water　soluble,　acid　stable　and　thermostable.　Therefore,　STX　in　seawater　can　be　accumulated　by　marine　organism　to　form　bioaccumulation.　To　ensure　the　safety　of　seafood　for　consumption,　it　is　crucial　to　accurately　determine　trace　STX　in　seawater　and　seafood.　We　herein　developed　a　novel　magnetic　electrochemical　immunosensor　for　ultra-sensitive　detection　of　STX　in　seawater　and　seafood　by　using　non-competitive　strategy.　The　immunosensor　employs　STX-specific　antibody-functionalized　magnetic　beads　(MBs)　for　STX　recognition,　palladium-doped　graphitic　carbon　nitride　(g-C3N4-PdNPs)　peroxidase　mimetic　for　catalyzing　H2O2-mediated　oxidation　of　3,3＇,5,5＇-tetramethylbenzidine　(TMB)　to　generate　signal.　The　immunosensor　combines　the　merits　of　g-C3N4-PdNPs　peroxidase　mimetic,　non-competitive　strategy,　MBs-based　antibody　recognition　and　magnetic　gold　electrode,　and　thus　has　excellent　stability,　lower　cost,　no　risk　of　false　positive　result,　high　sensitivity　and　strong　ability　resist　to　matrix　interference.　The　proposed　immunosensor　has　been　successfully　used　to　detect　trace　STX　in　seawater　and　shellfish　samples　with　a　detection　limit　of　1.2　pg/mL　(4.0　x　10(-12)　M),　a　recovery　of　93-107%　and　a　relative　standard　deviation　(RSD,　n　=　5)　＜　5%.　The　success　of　this　study　provided　a　promising　approach　for　the　rapid　and　on-site　detection　of　trace　STX　in　seawater　and　seafood.