Tungsten-based　materials　are　very　promising　anode　materials　for　lithium-ion　batteries.　Their　lithiation　processes　involve　two　types　of　reactions,　intercalation　and　conversion　reactions,　which　occur　sequentially　according　to　the　degree　of　lithiation.　However,　the　boundary　between　these　two　reactions　and　their　effects　on　the　overall　electrochemical　performance　still　need　more　in-depth　understanding　and　description.　Herein,　an　X-ray　diffraction　and　first-principles　calculation　were　combinedly　implemented　to　reveal　the　lithiation　processes　of　the　monoclinic　W18O49.　The　transformation　between　intercalation　and　conversion　reactions　was　found　to　be　dependent　on　the　number　of　intercalated　Li　atoms.　The　in-situ　XRD　analysis　shows　structural　phase　transformations　occurred　as　the　deep　charge-discharge　was　performed,　which　corresponding　to　the　structure　of　LixW18O49　(x　＞　22)　revealed　by　the　first-principles　calculation.　Moreover,　excellent　stability　of　battery　capacity　was　found　in　the　shallow　charge-discharge,　whereas　not　in　the　deep　charge-discharge.　Based　on　this　study,　we　comprehensively　deciphered　the　factors　that　affect　the　electrochemical　performance　and　reaction　mechanism　of　W18O49　with　lithium.　We　hope　this　study　could　contribute　to　the　application　of　W18O49　and　similar　transition　metal　oxide　anode　materials　for　lithium　ion　batteries.