Body　and/or　caudal　fin　(BCF)　fish　mainly　use　their　body　and　tail　fin　as　propulsors,　and　tune　their　stiffness　during　swimming　to　enable　rapid　and　efficient　locomotion.　However,　current　variable-stiffness　biomimetic　robotic　fish　with　high-frequency　actuation　mainly　focuses　on　the　effect　of　tail　fin　stiffness.　In　this　paper,　we　develop　a　free-swimming　tensegrity　robotic　fish　with　multi-tensegrity　joints,　to　experimentally　study　the　effect　of　online　body　stiffness　variation　in　fish-like　swimming　with　high　actuation　frequency.　We　detail　its　remote　high-frequency　driving　mechanism　and　fast　stiffness　adjustment　system.　We　validate　the　wide-range　and　fast　stiffness　adjustment　for　the　tensegrity　joint.　The　robotic　fish　can　dynamically　alter　various　body　stiffness　distributions　online　by　changing　its　joints’　stiffness.　The　experimental　results　illustrate　the　nonlinear　and　dramatic　effects　of　the　driving　frequency,　body　stiffness,　and　swimming　state.　The　ability　to　adjust　body　stiffness　online　in　swimming　is　demonstrated,　enabling　large　range　and　fast　changes　in　swimming　speed　and　thrust.　Compared　to　other　biomimetic　robotic　fish,　the　tensegrity　robotic　fish＇s　swimming　performance　is　at　an　upper-middle　level,　and　its　variable　stiffness　ability　is　outstanding.　This　work　offers　valuable　insights　for　the　future　optimization　online　of　the　swimming　process　in　biomimetic　fish　design.　©　2025　Elsevier　Ltd