A　thermal　cloak　is　well　known　for　hiding　objects　from　thermal　signature.　A　bilayer　thermal　cloak　made　from　inner　insulation　layer　and　outer　isotropic　homogeneous　layer　could　realize　such　thermal　protection.　However,　its　thermal　protection　performance　can　be　suppressed　for　low-thermal-conductivity　surrounding　media.　We　propose　a　tri-layer　thermal　cloak　model　by　adding　a　transition　layer　between　the　insulation　layer　and　the　outer　layer.　Numerical　simulations　and　theoretical　analysis　show　that,　under　the　same　geometry　size　and　surrounding　thermal　conductivity,　the　performance　of　the　thermal　cloak　can　be　significantly　enhanced　by　introducing　a　transition　layer　with　higher　thermal　conductivity　and　an　outer-layer　with　lower　thermal　conductivity.　The　tri-layer　cloak　proposed　provides　a　design　guidance　to　realize　better　thermal　protection　using　isotropic　bulk　materials.