Nowadays,　many　cities　in　China　are　facing　the　challenge　of　returning　black　and　odorous　waters　during　the　rainy　seasons.　Combined　sewer　overflow　(CSO)　was　one　of　the　critical　triggers.　In　this　study,　a　new　method　for　estimating　frequency　distribution　of　the　excess　rainfall　intensity　considering　double　thresholds　(ERId　in　short)　was　proposed,　which　is　helpful　for　estimating　severity　of　CSO.　For　the　specific　interceptor　well,　the　upper　threshold　is　the　design　areal　unit　discharge　(the　equivalent　rainfall　intensity)　of　its　upstream　combined　sewer　and　the　lower　one　is　the　design　areal　unit　discharge　of　its　downstream　interceptor　sewers　minus　dry　weather　flow　rate.　First,　rainfall　events　were　divided　and　the　minimum　inter-event　time　(MIET)　was　set　based　on　the　time　of　concentration　of　the　combined　sewer　catchment.　Second,　double-threshold　method　was　used　for　ERId　calculation　and　sampling　for　scenario　of　30-　minute　CSO　duration.　Finally,　the　empirical　frequency　of　the　sampled　ERId(30)　was　calculated　by　the　mathematical　expectation　formula　(Weibull　formula)　and　five　functions　(doubly　censored　exponential　distribution,　exponential　function,　Gumbel　distribution,　Weibull　distribution　and　lognormal　distribution)　were　used　for　comparison　to　obtain　the　best　theoretical　frequency　distribution.　Results　revealed　that:　(1)　The　excess　rainfall　intensity　of　double　thresholds　(ERId)　is　suitable　for　characterizing　CSO.　(2)　The　frequency　distribution　of　ERId　was　right　(positively)　skewed.　(3)　The　kurtosis　of　the　ERId　samples　is　not　greater　than　3　for　all　the　thresholds　studied,　which　indicates　that　the　distribution　is　thin-tailed.　(4)　The　optimal　frequency　distribution　function　for　ERId　is　doubly　censored　exponential　distribution.