The　electrochromic　WO3　devices　normally　suffer　from　the　degradation　of　long-term　performances　since　the　accumulation　of　trapped　ions　in　the　host　structure.　In　this　study,　we　notice　that　the　degradation　of　WO3　film　consists　of　the　decrease　of　bleaching　state　and　the　decay　of　coloration　state,　and　the　latter　is　the　main　factor　of　affecting　the　cyclic　stability　of　the　WO3　film.　Here,　it　is　discovered　that　the　degradation　of　coloration　state　is　due　to　the　decrease　of　the　charge　density　of　inserted　ions　in　the　film　during　cycling　process.　By　employing　the　open　circuit　potential　(OCP)　measurement,　we　find　that　the　accumulation　of　trapped　ions　in　WO3　film　will　cause　the　decay　of　OCP　between　the　work　electrode　of　WO3　film　and　counter　electrode,　which　is　the　main　reason　of　the　decrease　of　inserted　ions.　This　is　because　that　the　decay　of　OCP　will　weaken　the　electric　field　force　used　to　drive　ions　into　the　film　and　shorten　the　duration　time　of　ion　insertion　process.　Consequently,　the　decline　of　charge　density　of　the　inserted　ions　will　cause　the　decay　of　coloration　state,　which　is　the　real　mechanism　of　trapped　ions　eroding　the　cyclic　life　of　the　WO3　film.