In　flexible　electronic　systems,　mechanical　deformation　is　a　key　factor　that　impacts　the　mechanical　stability　of　flexible　devices.　In　this　work,　for　the　first　time,　a　mathematical　analysis　model　was　introduced　to　precisely　calculate　the　position　of　the　bending　axis　(the　strains　are　zero)　in　a　flexible　organic　thin　film　transistor　(OTFT)　system,　which　is　also　broadly　applicable　for　flexible　electronic　systems.　Based　on　this　mathematical　analysis　model,　a　universal　method　was　proposed　to　precisely　move　the　bending　axis　to　different　positions　in　a　flexible　OTFT　by　simply　adjusting　the　thickness　of　the　passivation　layer.　Moreover,　the　results　firstly　demonstrated　that　the　flexible　OTFT　exhibited　the　best　mechanical　stability　and　electrical　performance　when　the　bending　axis　was　located　at　the　gate　dielectric/semiconductor　charge　channel　interface,　as　the　charge　transport　was　mainly　at　the　charge　channel　interface,　and　moving　the　bending　axis　to　the　gate　dielectric/semiconductor　interface　would　minimize　the　impact　of　mechanical　deformation　on　the　charge　transport.　More　importantly,　this　work　definitely　demonstrated　the　importance　of　the　passivation　layer,　which　could　not　only　prevent　the　damage　caused　by　water　and　oxygen　in　the　air　to　flexible　devices,　but　could　also　tune　the　position　of　the　bending　axis　and　improve　the　mechanical　stability　of　flexible　devices　without　sacrificing　the　electrical　performance　or　increasing　the　complexity　of　the　fabrication　process.　Furthermore,　this　method　is　also　applicable　to　other　flexible　electronic　systems　and　has　great　potential　to　fabricate　flexible　electronics　with　excellent　mechanical　properties　and　environmental　durability.　©　2019　The　Royal　Society　of　Chemistry.