The invention discloses a fluorescent reagent for detecting trace serum albumin, and a preparation and application of the fluorescent reagent, and belongs to the field of fluorescent bio-sensors. The fluorescent reagent is prepared from DP-TPPNa and a good solvent. The preparation method comprises the following steps: dissolving 3,4-dibromo-1,2,5-tri((methoxycarbonyl)phenyl) pyrrole in the good solvent, adding phenylboronic acid, a catalyst and basic salt, and reacting to obtain DP-TPP; dissolving the DP-TPP in a solvent, adding alkali, and reacting to obtain DP-TPPNa; and dissolving the DP-TPPNa in the good solvent to obtain the fluorescent reagent. The fluorescent reagent is capable of quantitatively detecting the serum albumin in serum, is simple in preparation method, convenient to operate and quick in response, has high sensitivity and high selectivity, is capable of producing an excellent visible detection signal, can completely get rid of the dependence on high-precision instruments according to actual requirements, and can meet the requirement of people on detecting the serum albumin in serum today.