A class of coumarin-oxacalix[3]arene fluorescent reagent and its preparation and application of the present invention belong to the field of organic synthesis and analytical chemistry. The present invention synthesizes two kinds of coumarin-oxacalix[3]arene fluorescence reagents respectively Reagent, chemical name a : 7,15,23-tri-tert-butyl-25,26,27-tri{1-[N-(7-coumarinoxyethyl)-1,2,3-triazole]-4-methyl Oxy}-3,11,19-trioxacalix[3]arene; b : 7,15,23-triethoxycarbonyl-25,26,27-tri{1-[N-(7-coumarinoxyethyl)-1,2,3-triazole]-4-methanol Oxygen}-3,11,19-trioxacalix[3]arene, and discloses the synthesis method and process conditions, the present invention uses two kinds of coumarin-oxacalix[3]arene fluorescent reagents to detect three Valence iron ion, with high selectivity and high sensitivity, can be used for qualitative and quantitative analysis.