The　proper　setting　of　threshold　is　crucial　for　the　early　prediction　of　the　short　circuit　fault　in　the　low-voltage　distributed　system　when　using　the　method　of　wavelet　decomposition.　The　approach　of　threshold　setting　for　the　short　circuit　fault　early　detection　based　on　quadratic　B　spline　wavelet　has　been　discussed　in　this　paper　by　researching　on　the　transient　process　of　motor　starting,　overload　and　short　circuit　faults　in　low-voltage　system.　Compared　with　the　Full-wave　Fourier　Module　method,　this　approach　can　distinguish　the　different　types　of　fault　and　detect　the　fault　quickly.　The　validity　of　this　method　has　been　verified　by　simulating　results.