To　face　the　challenge　of　adapting　to　complex　terrains　and　environments,　we　develop　a　novel　wheel-legged　robot　that　can　switch　motion　modes　to　adapt　to　different　environments.　The　robot　can　perform　efficient　and　stable　upright　balanced　locomotion　on　flat　roads　and　flexible　crawling　in　low　and　narrow　passages.　For　passing　through　low　and　narrow　passages,　we　propose　a　crawling　motion　control　strategy　and　methods　for　transitioning　between　locomotion　modes　of　wheel-legged　robots.　In　practical　applications,　the　smooth　transition　between　the　two　motion　modes　is　challenging.　By　optimizing　the　gravity　work　of　the　body,　the　optimal　trajectory　of　the　center　of　mass　(CoM)　for　the　transition　from　standing　to　crawling　is　obtained.　By　constructing　and　solving　an　optimization　problem　regarding　the　posture　and　motion　trajectories　of　the　underactuated　model,　the　robot　achieves　a　smooth　transition　from　crawling　to　standing.　In　experiments,　the　wheel-legged　robot　successfully　transitioned　between　the　crawling　mode　and　the　upright　balanced　moving　mode　and　flexibly　passed　a　low　and　narrow　passage.　Consequently,　the　effectiveness　of　the　control　strategies　and　algorithms　proposed　in　this　paper　are　verified　by　experiments.　©　2022　by　the　authors.