This　paper　proposes　a　phase　detection　method　for　active　rectification　of　mega-hertz　wireless　power　transfer　systems.　By　using　a　decoupled　auxiliary　coil　integrated　on　the　receiving　coil,　the　switching　phase　of　active　rectifiers　can　be　detected　indirectly.　The　auxiliary　coil　has　much　higher　available　frequency　compared　with　conventional　methods　using　current　transformers.　Analytical　modeling　and　implementation　of　decoupling　coil　structures　are　investigated.　The　interference　from　the　receiving　coil　current　can　be　well　suppressed　due　to　the　decoupling　feature,　thus　the　auxiliary　coil　acts　as　a　mapped　transmitting　coil,　which　realizes　high-stability　phase　detection　and　position　sensing　for　the　receiver.　A　6.78-MHz　receiver　with　active　class　E　rectification　is　built　to　verify　the　proposed　methodology,　which　achieves　phase　detection　and　misalignment　measurement　with　high　stability,　low　cost,　and　compact　size.　The　proposed　method　is　preferable　for　wireless　power　transfer　applications　with　mega-hertz　active　rectifiers.　©　2022　IEEE.