Variable　stiffness　actuator　(VSA)　is　important　for　its　contribution　to　improve　the　robot＇s　performance　and　safety,　especially　in　case　of　interactions　between　human　and　uncertain　environments.　In　this　paper,　a　VSA　is　designed　based　on　beam　flexure　as　well　bistable　compliant　crank-slider　mechanism.　The　beam　flexure　and　the　bistable　mechanism　are　connected　by　a　slider,　offering　a　variable　supporting　force.　The　stiffness　is　not　influenced　by　the　joint　position　and　the　joint　orientation,　but　associated　with　the　distance　from　the　slider　to　rotating　center　of　variable　stiffness　mechanism.　This　paper　illustrates　the　operating　principle,　with　a　mechanics　model,　and　validates　the　characteristics　by　experiments.　©　2019　IEEE.