The invention discloses a rhodamine B-based fluorescence sensor and preparation. The preparation comprises the following steps: dissolving 3-(3',6'-bi(diethyl amino)-3-oxaspiro[isoindoline-1,-1,9'-xanthene]-2-yl) propanal (A) in methyl alcohol, adding 6-aminocaproic acid (B), and stirring at room temperature; dissolving sodium triacetoxyborohydride in tetrahydrofuran, adding to a reaction liquid in the step 1, and further carrying out stirring reaction at room temperature; dropwise adding a saturated sodium bicarbonate solution to the reaction liquid, and further stirring; and removing a solvent by virtue of a rotary evaporator, and separating by using a thin layer chromatography, so as to obtain a target product. The rhodamine B-based fluorescence sensor is relatively simple in synthesis step; resources are saved; and nearly all the reported fluorescence probes can only be applied to an organic solvent or an organic water solution, thus the application of the fluorescence sensor in the fields of actual water sample analysis and life is limited.