Planar　spiral　coil　made　of　solid　wire　is　widely　used　in　wireless　power　transfer　without　magnetic　core　in　high　frequency　operation　system.　Based　on　the　magnetic　field　distribution　of　planar　spiral　coil　and　high　frequency　eddy　current　loss　mechanism,　the　influence　of　magnetic　field　strength,　frequency,　and　wire　gauge　on　the　AC　resistance　of　winding　are　analysed.　It　reveals　that　the　coil　loss　can　be　reduced　by　designing　the　wire　gauge.　Then　the　magnetic　field　strength　and　the　winding　loss　calculation　model　are　built,　based　on　which　the　double　wire　gauge　for　segmental　winding　structure　is　proposed.　This　method　can　reduce　the　AC　resistance　and　improve　the　quality　factor　on　the　premise　of　keeping　inductance　constant　at　high　frequency.　Through　simulation　and　experiment,　the　AC　resistance　of　coil　based　on　the　double　wire　gauge　scheme　can　be　greatly　reduced　comparing　to　the　traditional　method　and　the　efficiency　of　the　prototype　is　increased　by　4%,　which　verifies　the　theory　analysis　to　be　correct　and　flexible.