The invention discloses a dendritic fluorescent chemical sensor, and its preparation method and application, and also discloses a method for identifying and detecting Pd <2+> through the dendritic fluorescent chemical sensor. The invention belongs to the field of organic synthesis and analytical chemistry. The dendritic fluorescent chemical sensor is characterized in that rhodamine B hydrazide as a fluorescent active site is introduced into a dendritic compound to form a novel compound, wherein a structural formula of the novel compound is shown in the patent specification. The preparation method of the dendritic fluorescent chemical sensor comprises the following steps of in an argon atmosphere, putting an aldehyde compound with a quadrilateral molecular configuration into a reactor, adding ethanol into the aldehyde compound to make the compound dissolve, adding dropwisely rhodamine B hydrazide-ethanol solution into the aldehyde compound-ethanol solution with stirring under a constant pressure, wherein a mol ratio of the added aldehyde compound to the added rhodamine B hydrazide is 1: (4.2 to 4.5), when a adding process of the rhodamine B hydrazide-ethanol solution is finished, adding 1.0 to 2.0% by volume of glacial acetic acid into the mixed solution in the reactor, stirring well, carrying out a reflux reaction at a temperature of 100 DEG C for 18 to 22 hours, evaporating the resulting products to remove a solvent and collect residue, washing the residue 3 to 5 times by ethanol, and drying in a vacuum to obtain a dendritic fluorescent chemical sensor product which is a pale pink solid. The dendritic fluorescent chemical sensor prepared by the preparation method has four fluorescent active groups thus a sensing speed and a sensing sensitivity of the dendritic fluorescent chemical sensor is 4 times as high as a sensing speed and a sensing sensitivity of a small molecule. The dendritic fluorescent chemical sensor does not contain toxic solvents.