The　AC　line　transients　are　inevitable　during　the　operation　of　the　power　factor　correction　(PFC).　For　2-level　PFCs,　such　transients　can　often　pass　smoothly　without　protection,　as　the　large　inductance　of　input　filters　can　store　enough　energy　to　withstand　short-term　abnormalities.　However,　for　the　flying　capacitor　multilevel　(FCML)　PFC　with　a　significantly　smaller　inductance,　AC　line　transients　could　lead　to　rapid　over-current　issues　and　ruin　the　voltage　balance　of　flying　capacitors　(FCs),　resulting　in　over-voltage　across　switching　devices　and　endangering　the　integrity　of　the　converter.　Among　all　different　kinds　of　AC　line　transients,　the　AC　drop　transient　is　the　most　challenging　one,　in　which　the　grid-side　voltage　suddenly　drops　to　0　and　last　for　a　certain　period　of　time.　In　light　of　this　issue,　this　paper　presents　a　strategy　for　fast　protection　and　quick　recovery　of　the　FCML　PFC　under　the　AC　drop　transient.　It　also　analyzes　the　oscillation　dynamics　of　the　flying　capacitor　voltages　(FCVs)　during　the　recovery　process　and　subsequently　calculates　the　safe　operating　threshold　of　the　switching　devices.　Finally,　through　a　five-level　FCML　totem-pole　prototype,　this　paper　experimentally　validates　the　feasibility　of　proposed　PFC　protection　and　recovery　strategy.　Furthermore,　the　oscillation　dynamics　of　FCV　is　also　tested　under　dc/dc　boost　mode.　©　2025　IEEE.