To　ensure　the　rationality　of　rainwater　tank　design,　we　need　to　deduce　rainfall　depth　formula.　Apply　the　latest　fifty　years　daily　rainfall　data　of　the　city　of　Beijing,　based　on　the　theory　of　probability　analysis　and　statistics,　four　candidate　distributions　(Three-Parameter　Lognormal,　Pearson　Type　III,　Generalized　Extreme　Value　and　Two-Parameter　Exponential)　were　used　to　fit　the　observed　data　and　find　the　most　appropriate　distribution.　The　annual　multi-sampling　series　method　was　used　to　process　rainfall　depth　data.　The　assessment　was　based　on　the　Kolmogorov-Smirnov　test　combined　with　maximum　absolute　error.　Finally　the　expression　of　the　rainfall　depth　formula　was　obtained.　On　the　basis　of　estimation　results,　it　can　be　concluded　that,　four　distributions　do　seem　to　fit　the　data　sets　well.　The　Three-Parameter　Lognormal　distribution　is　the　most　appropriate　distribution　for　describing　the　annual　multi-sampling　rainfall　series.　The　rainfall　depth　formula　is　given.　The　method　of　deriving　rainfall　depth　formula　may　be　employed　to　other　city　or　region.　Rainfall　depth　formula　with　determined　parameters　may　use　to　generate　daily　rainfall　depths　with　different　recurrence　intervals.