Wireless　power　transfer　(WPT)　technology　has　been　widely　studied　and　used　in　academia　and　industry.　However,　the　system＇s　efficiency　is　still　the　bottleneck　of　WPT　technology.　Moreover,　the　magnetic　coupling　system＇s　loss　usually　occupies　much　of　the　total　loss.　Furthermore,　the　magnetic　coupling　system＇s　core　loss　is　hard　to　model　and　optimize　due　to　the　uneven　flux　density　distribution　in　the　magnetic　core.　Therefore,　this　paper　analyzes　the　magnetic　flux　density　inside　the　magnetic　coupling　system＇s　magnetic　core　with　a　spiral　winding　structure.　It　reveals　that　the　flux　density　is　quite　uneven　in　the　magnetic　core,　and　the　core　loss　dominates　where　the　flux　density　peaks.　Based　on　it,　a　new　varied　section　of　the　magnetic　core　structure　is　proposed.　An　approximate　even　flux　density　distribution　can　be　obtained　with　the　proposed　structure,　and　37.14%　can　greatly　reduce　the　core　loss.　Considering　the　proposed　method＇s　manufacturing　difficulty,　a　double-section　improvement　method　of　the　magnetic　core　structure　is　proposed.　With　the　double-section　improvement　method,　the　flux　density　can　be　evener　compared　to　the　traditional　unique　section　method.　The　core　loss　can　be　effectively　reduced.　Finally,　a　3kW　electric　vehicle＇s　WPT　system　is　taken　for　experiments.　The　double-section　method　can　improve　the　total　efficiency　by　0.8%,　the　machine　maximum　efficiency　reaches　95.3%.　It　verifies　the　proposed　method　to　be　correct　and　useful.　©　2023,　The　Author(s),　under　exclusive　license　to　Springer　Nature　Singapore　Pte　Ltd.