The　distributed　power　supply　decentralized　access　to　the　low-voltage　side　of　the　distribution　network　transforms　the　existing　low-voltage　AC　system　into　an　active　low-voltage　AC　system.　The　original　low-voltage　AC　short-circuit　detection　methods　are　no　longer　applicable.　In　this　paper,　a　short-circuit　early　detection　method　suit-able　for　different　branch　types　in　the　active　low-voltage　AC　system　is　proposed.　Based　on　the　analysis　of　the　transient　characteristics　of　the　active　low-voltage　AC　short-circuit,　a　short-circuit　time-varying　vector　theory　and　a　fast　method　for　solving　its　modulus　length　are　proposed.　Then,　we　construct　a　branch　state　variable　containing　time-varying　vector　modulus　length　difference　information　of　current　and　voltage.　The　constructed　branch　state　variable　can　reflect　the　characteristics　of　short-circuit　state　in　real-time,　which　greatly　improving　the　reliability　of　fast　short-circuit　detection.　Finally,　the　early　detection　of　short-circuit　fault　can　be　realized　by　comparing　the　branch　state　variable　with　the　set　detection　threshold.　The　physical　experiment　system　experiments　show　that　the　proposed　short-circuit　early　detection　method　is　effective　and　reliable.　Compared　with　the　existing　short-circuit　fast　detection　method,　the　detection　time　is　reduced　by　3-5　ms.　Moreover,　the　short-circuit　breaking　and　current　limiting　can　be　achieved　by　combining　with　the　fast　protection　mechanism,　and　the　current　limiting　ratio　is　as　low　as　0.33.