Flexible　electrochromic　devices　(ECDs)　have　generated　widespread　interest　due　to　their　attractive　application　prospects　in　emerging　wearable　and　portable　electronics,　electronic　papers/billboards,　and　other　advanced　display　applications.　However,　the　lack　of　rational　design　for　flexible　electrochromic　devices　with　both　high　flexibility　and　ultrathin　nature　remains　a　challenge,　limiting　its　development　toward　practical　applications.　Herein,　a　flexible　and　ultrathin　ion-conducting　film　(ICF)　is　proposed　by　combining　poly(vinylidene　fluoride)　polymer　matrix　with　electrolyte　to　assemble　into　multicolor　electrochromic　devices.　As　a　result,　the　as-obtained　ICF　exhibits　extremely　high　visible-range　transmittance　of　over　90%,　ionic　conductivity　of　0.11　mS　cm(-1)　at　room　temperature,　large　electrochemical　window　(3.55　V),　and　excellent　mechanical　flexibility,　making　it　an　excellent　candidate　for　the　electrochromic　applications.　By　taking　advantage　of　the　ion-conducting　film,　the　fabricated　ICF-based　electrochromic　device　based　on　polyaniline　(PANI)　electrode　exhibits　reversible　color　transformations　from　chartreuse　to　green　and　dark　blue,　as　well　as　ultrathin　characteristics,　excellent　flexibility,　and　strong　mechanical　stability,　superior　to　the　behavior　of　the　ECDs　fabricated　with　traditional　gel　electrolytes.　The　electrochromic　behavior　in　the　PANI　during　the　electrochemical　process　is　comprehensively　studied　by　density　functional　theory　(DFT)　calculations.　In　addition,　the　device　also　shows　an　effective　electrochromism　with　multicolor　properties　at　a　wide　temperature　range　(-10　to　60　degrees　C).　This　work　provides　a　feasible　way　for　assembling　high-performance　flexible　ECDs　for　potential　applications　in　multicolor　displays　and　wearable　electronics.