To　increase　the　power　density　of　a　wireless　power　transfer　(WPT)　system,　the　coupling　of　coils　can　be　strong.　However,　in　a　strongly　coupled　series-series　WPT　system　with　a　diode　rectifier,　currents　can　be　distorted　and　discontinuous,　and　first　harmonic　approximation　is　no　longer　valid.　The　inverter　is　likely　to　work　in　hard　switching,　leading　to　extra　noise,　decreased　efficiency,　and　a　potential　hazard　for　a　safe　operation.　Through　time-domain　analysis,　this　article　reveals　that　with　a　dual-side　90　degrees　phase　shift,　the　hard　switching　of　the　inverter　can　be　avoided,　the　power　level　can　be　enhanced,　and　the　discontinuous　conduction　mode　can　be　avoided　to　eliminate　noises.　The　factors　impacting　the　soft　switching　operations　of　the　inverter　and　the　rectifier　are　analyzed.　To　achieve　90　degrees　dual-side　phase　shift,　the　conventional　approach　of　using　the　receiver　voltage/current,　which　can　be　distorted　and　discontinuous　in　strongly　coupled　WPT　systems,　for　phase　synchronization　is　no　longer　suitable.　A　new　set　of　coils,　namely　driving　coils,　is　used　for　phase　synchronization　so　that　dual-side　phase　shift　can　be　implemented　independent　from　power　transfer.　Experimental　results　have　validated　the　effectiveness　of　the　analysis.