A　vibration　isolator　based　on　a　tunable　negative　stiffness　mechanism　combines　the　advantages　of　high-staticlow-dynamic　stiffness　(HSLDS)　isolators　to　expand　the　isolation　frequency　band　and　variable　stiffness　isolators　to　suppress　resonance.　In　this　paper,　a　novel　tunable　negative　stiffness　spring　using　Maxwell　normal　stress　(SMNS)　is　proposed.　The　stiffness　tunable　range　and　energy　utilization　efficiency　are　greatly　improved　due　to　the　newly　designed　magnetic　circuit.　Moreover,　the　electromagnetic　negative　stiffness　device　has　the　advantages　of　no　friction,　no　backlash,　compact　structure　and　easy　control.　An　analytical　model　of　the　electromagnetic　force　is　built　based　on　magnetic　circuit　analysis,　and　the　parameter　analysis　is　performed.　An　HSLDS　isolator　is　constructed　by　connecting　the　SMNS　in　parallel　with　a　linear　isolator.　The　stiffness　and　vibration　isolation　performance　are　measured.　The　experimental　results　show　that　the　SMNS　produces　an　online　tunable　negative　stiffness,　which　expands　the　isolation　bandwidth　and　significantly　improves　the　vibration　isolation　performance.